Chemistry 353 Midterm - Learnfaster.calearnfaster.ca/wp-content/uploads/chem353/Chem353MT-Lecture20… · Lecture Slides Booklet Solutions Test ... -2,4-dibromo-3-methylpentane has

Chemistry 353 � Midterm 1

(C) Pavel SedachLearnfaster.ca

Questions? Suggestions? E-mail me and I will try to find time to reply. [email protected]

All the best!

� Lecture Slides� Booklet Solutions� Test Solutions� Extra notes and advice� Advice on which exams and questions to write� Corrections

Available March 4th at:http://learnfaster.ca/blog/chem-353-midterm/

Saturday, March 4

2:00 pm to 3:30Alkenes and Alkynes, Chirality and

Stereoselectivity

3:30 to 3:50 Break

3:50 to 5:00 Substitution, Epoxidation

Sunday, March 5

11:00 am to 12:30 pm Ozonolysis, Diels Alder, Special Reactions

12:30 to 1:00 pm Break

1:00 to 2:00 Review/Problem Solving

Solitions will be posted at www.prep101.com/solutions

Naming Alkenes 2


cis/trans

longest carbon chain

E/Zlargest atom (Cahn Ingold Prelog Rules)

neither


Trends in Alkene/Alkyne Reactivity 3

Alkane Stability - more branched means more stable (assuming constitutional isomers)

Stability

> >

Reaction Coordinate

Heat of Combustion

Ener

gy

��

Heat of Combustion



Alkene Stability: trans > cis and branched > unbranched 4


Other Trends: Conjugated > Isolated > Cumulated

Heats of Combustion

E or transZ or cis

Stability

> >


Review W2016Q9 and W2016Q29 5


What is the relative rate of reactivity of the following alkenes with ��? (least stable = most reactive)

��

��

Arrange the following isomers from most stable to least stable:

s-cis s-trans


Elimination Mechanisms: �� - Anti-Elimination produces the most stable alkene (Zaitsev) 6


Zaitsev

Anti-Zaitsev

��

��heat

or��

heat


Review 7


What is the relative quantity of product formed by the reaction of 2-bromo-3-methylbutane with each of the following:

ii iii

��

��

i

��

What is the relative rate of reaction of R-2-bromo-3-methylbutane with each of the following:

ii iiii

��


Hyperconjugation vs. Resonance

Radical and Carbocation Stability 8


Classification?e.g. ��

Allylic or Vinyllic?

�� move via � bonds �� move via � bonds

+

+ + ++

+

+

MOST STABLE LEAST STABLE

benzylic > allylic > neither > vinyllic > vinyllic > phenylic


Hydrolysis of Alkene 9


The hydrolysis occurs through adding an H followed by an OH

��

akadilute ��

�� 2) 2) ��

Markovnikov � possibility of rearrangement (HEAT)*Performing reaction in the �Cold� may stop rearrangement.Occurs through a carbocation so stereochemistry is racemicReplacing �� with an alcohol makes ethers

Anti-Markovnikov � no possibility of rearrangementOccurs through syn addition*Watch out! 1 molecule of �� reacts for every 3 substrates. Replacing NaOH with NaOR (alkoxide) makes ethers

�� ensure a Markovnikov hydrolysis with no rearrangement. Occurs through anti additionReplacing �� with alcohols makes ethers

�� OR �� /��

�� N��

ANY of the above reactions can be used to make ethers!


Rearrangement with Sulfuric Acid 10


When a strong acid is used, rearrangement becomes a possibility because the intermediate is a carbocation. A rearrangement occurs because a more stable (thermodynamic) product can be produced rather than the more accessible (less steps/effort -kinetic) product:

��

��

��

Heat (Δ)

��

kinetic product

thermodynamic product

Ener

gy low Ea

high Ea

Rxn Coordinate

Cold


Review: Rearrangement with Sulfuric Acid W2016Q11 11


��

��

There are two likely products for the following reaction. Draw a mechanism for reaching both.


Review: Hydration with Boron 12


Syn addition occurs because, initially, B and H add on to one side of double bondBBN9 is a bulky analogue of �� - more effectively ensures anti-Markovnikov, only has 1 H to react.

�� or ��2) sodium isopropoxide��

1) BBN-9 2) N��

�� reacts in a 1:3 ratio (it has 3 Hs to give!) with your substrate. �� stoichiometry questions can be asked.

1) ��

2) ��


Bond Strength 13


Bond Strength and Bond Dissociation Energy (Bond Energy)

Bond Length

<

> Double Bond Triple Bond

Classification?e.g. ��

Allylic or Vinyllic?

C-H Bond Strength (opposite of radical stability)

Single Bond

<

>

��

allylic allylic allylicneither neither neither vinyllic


Review W2016Q10 and W2016Q26 14


What is the relative length of the indicated CC bonds?

i

iii

ii

iv

Which of the following systems are conjugated (select all that apply)?

*A � bond next to an �� hybridized carbon is conjugated. Anions can also be conjugated to � bonds.

a b c d e


Chem 351 was all about trends - 353 is all about synthesis! Whenever possible, avoid radical substitution � it is NOT efficient!

Relative Yield in a Radical Reaction 15(C) Pavel SedachLearnfaster.ca

Reactivity Factors

��/uv

Br Cl F

1° 1 1 1

2° 82 3.9 1.2

3° 1640 5.2 1.4

most selectiveleast selective

Number of Hydrogens

Selectivity Factor

Relative Yield:

Total


Whenever possible, avoid radical substitution� Unless the product is obvious/limited. Additionally, most syntheses state you canstart with ANY hydrocarbons 3 atoms or less � this includes ALKENES and ALKYNES � use those instead!

Tell me the possible products and their relative yield 16


Reactivity Factors

��/uv

Br Cl F

1° 1 1 1

2° 82 3.9 1.2

3° 1640 5.2 1.4

��/uv

��/uv

Alternative synthesis of 1-bromobutane from an alkene?


Addition of HBr to Alkenes � The Most Stable Carbocation is Preferred 17


The addition of the H is to the side of the double bond with the most Hs (Markovnikov)

major

minor

� � ��

� � ��

� � ��


ReferenceCompound:

Electron Donating and Withdrawing Groups in Carbocation Stability 18


Electron Donating Groups and Electron Withdrawing Groups

If the atom next to the chosen position is Delta Negative and has a lone electron pair, it is a donor.

Alkyl groups are weakly donating.

If the atom next to the chosen position is Delta Positive, you are dealing with an Electron Withdrawing Group.

Alkenes/Alkynes are withdrawing groups BUT they stabilize carbocations through resonance.

withdrawing through induction, donating through resonance


What is the product of the following reactions? Give a mechanism. 19


If the mechanism forms a carbocation, there is always some possibility of rearrangement.

� � ��

geminal

(think �in the vicinity of�)

vicinal

� � ��

� � ��

� � ��peroxides

� � ��

The alkyne addition is concerted (1 step)


Summary of Ideal Conditions 20


If Heat is present in �� or �� it is often a que for rearrangement.. Why?

Reaction Solvent Nu or BaseLeaving Group

Substrate Example conditions

�� Polar ProticWeak Nu

Weak BaseVery

Good LG�� or resonance

stabilized

alkyl halide reacted with��/��

alcohol reacted with HX

�� Polar Aprotic�Good Nu but

Poor baseGood LG ��

alkyl halide reacted with NaI/acetonealcohol reacted with ��

alcohol reacted with ��

��Polar solventsCan be protic

Weak base Good LG�� or resonance

stabilized

alkyl halide reacted with ��

alcohol reacted with ��/heat

��Polar,

Ideally AproticStrong base but

Poor NuGood LG ��

alkyl halide reacted with KOH / heatH and LG MUST BE ANTI


Hydrogenation - addition of hydrogen 21


A noble metal catalyst such as Platinum, Palladium, Ruthenium or Rhodium is used along with pressure and heat.

Problem 1) both alkynes and alkenes get taken all the way to the alkane (no selectivity)Problem 2) The addition of hydrogen molecules is syn addition (to the same face)

Consequence:

��

��

��

Noble metal

This selectivity occurs because the reagents are attracted tothe surface of the metal catalyst.

Hence addition of hydrogen occurs only on one face.

assign R/S stereochemistry


Relative and Absolute Stereochemistry 22


relative invertedabsolute inverted��

�

�

�

R

relative invertedabsolute retained

R (Clockwise) or S (Counterclockwise)

i. Priority 1 is given to the largest atom at 1st point of difference (Cahn-Ingold Prelog Rules). Remaining groups are numbered 2 through 4 according to priority.

ii. The molecule is oriented so the group with the lowest priority (4) is pointing into the pageiii. The rotation of groups 1 through 3 gives either an R (Clockwise) or S (Counterclockwise) assignment


Assigning R/S 23


If the least important group is not pointing into the page, you can any two groups to get the opposite stereochemistry and assign as normal. Then, because you switched two groups, the original compound MUST have opposite stereochemistry.

If you flip any two groups you have the opposite stereochemistry. Try it out:

Least important group is in the plane of the page SO we switch it with the hash group

I assigned the structure at right, then used that information to assign the original structure at left.Because the result of flipping any two groups was R, the original must be S.


�

Enantiomers and Diastereomers 24


Enantiomers - complete mirror image - all stereocenters opposite

RR � SS SSSR�RRRS

Enantiomers have the same boiling point and other physical properties. They rotate light in the opposite direction.

Diastereomers have some (but not all) stereocenters flipped

SSSS � SSSRSSSS � RSSS

Unlike enantiomers, Diastereomers have DIFFERENT physical properties (e.g. boiling point).

Fischer Projections (arms come out to hug you!)

=

Newman Projections W2015Q21

��

� ��

��

��

��

��

��

For Newman projections, I cut the molecule down the middle and draw it as wedge hash.


Rotation of Plane Polarized Light W2015Q6 25


Enantiomers rotate plane polarized light in opposite directions to the same degree.

Given (2S,4S)-2,4-dibromo-3-methylpentane has a specific optical rotation ��, what is the specific optical rotation of the following samples?

50:50 50:50 Purei ii iii iv v

What is the relationship between compounds i and iii?

i and v?

Bring a scientific calculator � you may be asked to do mass analysis or percent yield


Isomers 26


Not Isomers

IsomersDo the compounds have the same molecular formula?

ConstitutionalStereoisomers

Can the compounds be interconverted by rotation about single bonds?

Conformational Configurational

Is the isomerism at a double bond?

Geometric

YESNO

YES NO

Are the atoms connected the same way?

YES NO

YES NO

Optical

Constitutional

Conformational

Geometric

Optical

Same formula (��), different skeleton!

trans-but-2-en-1-olOR

E-but-2-en-1-ol

cis-but-2-en-1-olOR

Z-but-2-en-1-ol

Rotate around single bond

DiastereomersEnantiomers

YES NO

Are the compounds non-superimposable mirror images?


Polarimetry: W2015 Q6 27


What is the specific rotation of the mixture below when 1.0 g of the mixture is placed in a standard 10 mL polarimeter cell?

��

�� RR SS

5.0 grams 17.0 grams

��

��

*observed rotation is usually the same as specific on test questions!

If the specific rotation of a different mixture of R,R and S,S-2,4-dibromo-3-methylpentane was ��, which enantiomer was in excess? What was the relative quantity of the two enantiomers?


Hydrogenation Continued 28


How do we deal with the complete hydrogenation of alkynes? We �poison� the catalyst!

Addition using a metal always creates the cis alkene due to syn addition

Note that Lindlar�s catalyst or poisoned Palladium only reacts with alkynes. Why is that?

��

(Lindlar�s catalyst)��


Hydrogenation 29


How do we make a selective trans hydrogenation of alkynes? We need to provide a mechanism for anti addition of ��

(Lindlar�s catalyst)��

Sodium metal reacts breaks the triple bond and forms a carbanion (a ��, yes, a carbon atom literally has a lone pair and a negative charge). This carbanion then react with ammonia and takes a hydrogen. The process repeats on the opposite side of the newly formed stable trans alkene.

Note: Do NOT confuse �� with ��. If you add �� to an alkyne that has an H, you deprotonate it and make it a good reagent for substitution. �� on the other hand reduces the alkyne to a trans alkene.

(sodium metal and ammonia)��

��

��


Review 30


��

��

��

��

��

��


Review 31


��

��

��

��


��

OR��

��

ORany peroxyacid

Hydroxylation using �� or �� 32


Notice that the addition of hydroxide with �� or �� is syn addition.

Epoxides and Hydrolysis of Epoxides:

The above is how an anti addition of vicinal hydroxides occurs. Any nucleophile can be used for the ring opening and an epoxide is a versatile, reactive reagent.

�� or ��

Assign R/S stereochemistry.

Assign R/S stereochemistry.


Review 33


What is the relationship between the products of this reaction? (Enantiomers, Diastereomers, Meso compounds)

��


Review 34


Which Newman projection represents one of the products of this reaction?

��

��

��

��

��

��

��

��

��

��

��

��

�

��

��

��


Substitution, Elimination and ��/�� like reactions © Pavel Sedach Learnfaster.ca

35


Summary

© Pavel Sedach Learnfaster.ca

36

Telltale reagents for ��:

When an alkyne or carbonyl is used with �� they�re justpreparation of a �� nucleophileCommon Polar Aprotic SolventsDMSO (dimethyl sulfoxide)

Acetone

Chloroform or any �� or �� derivative e.g. ��

Reaction conditions that lead to rearrangementHeat�� with Alcohol�� with AlcoholWhy does rearrangement occur? Long lived _________

Excellent ��

��

Good ��

�� Very Poor ��

Excellent �� , �� Good ��

Poor ��

Non-Leaving Groups ��

Nucleophiles Leaving Groups

Telltale reagents:�� used to remove LG for ��Na � used to drive equilibrium for ��TsCl � tosyl chloride - used to convert alcohols into EXCELLENT LGs for subsequent �� *(solvent is usu. pyridine)Why is �� preferred over �� sometimes? ______________Common Polar Protic SolventsWater

Ethanol

Acetic Acid

Solvents that are also reactantsWaterEthanolAmmonia

��


What is the relative withdrawing ability of the following substituents (in terms of the alkene)? 37


We typically consider �H as a standard/reference when discussing trends.Rank the following compounds in order of acidity (1 is most acidic, 7 is least acidic):

Rank the following compounds in order of electron withdrawing ability (1 is most withdrawing, 5 is most donating)


Acidity Review 38


1) Strength of the bond holding the hydrogen to the molecule

��

(Bigger atom = weaker bond = more acidic)

��

2) Polarity of the bond

(More electronegative atom in the same row = more acidic)��>��>��>��

(More electron withdrawing groups = more acidic)��

3) Stability of the conjugate base ��>��>��

��

��

pKa

Acidity 0

10

20

30

40

50

��

��

��

��

��

alkanes

��

Strong Acids

��(amide)


�� - 2 Step Reaction 39


� Goes through a carbocation intermediate rate determining step. If the reactant is chiral, the product will be racemic.

��

��/��

��

��

��

1st order overall so ��

��

Whenever there is a carbocation, there is an opportunity for rearrangement!

(S)-2-chlorobutane � �� (R)-2-iodobutane � ��

��

RDSacetone

��

1+1=2nd order overall so ��

No intermediate but a polar transition state occurs. The reactant undergoes inversion of stereochemistry � � � or � � ��

��2 - 1 Step Reaction


Review


40

If �� is such a good leaving group (in addition to being a good nucleophile), what drives the above reaction?

Nucleophilicity��

Leaving Group Ability��

Basicity��

Nucleophilicty � Basicity

BECAUSE polarizability (atomic size) is more important in nucleophilicity

Most stable base(poorest base)

Least stable base(best base)

��acetone

��acetone

What role does the solvent play?

NaCl and NaBr are insoluble in most polar aprotic solvents. We add �� containing reagents for ��.

Nu:substrate:LG:solvent:��/��?



Review


41

What role does �� play?

Nucleophilicity��

Leaving Group Ability��

Basicity��

Most stable base(poorest base)

Least stable base(best base)

��

��

��

What role does the solvent play?

AgCl, AgBr and AgI are insoluble in nearly all solvents. We add �� containing reagents for ��.





�� - 2 Steps 42


Goes through a carbocation intermediate RDS (rate determining step). Forms a double bond.

��

slowRDS

1st order overall so ��

sulfuric acid

hydrogen sulfate acting as a base

major

minor

�

�

Product based on stability of

carbocation


Elimination: Match the reaction to its product 43


1. TsCl2. NaBr3. KOH

��

1. TsCl2. NaBr3. KOtBu


Halogenation of Hydroxyl Groups with �� or �� is always ��


44

�� mechanism

Works on Primary �� and Secondary �� alcohols.

Tertiary alcohols �� do not generally react with these reagents. Instead, HX (X=halide) would be used to facilitate an ��1 reaction to generate an alkyl halide.

*�� is thionyl chloride.

Carboxylic Acids (*mechanism no longer ��)*RARE

�� or �� or ��

�� or ��

�� or �� or ��

�� or ��

pyridine (or ether)

pyridine (or ether)

pyridine (or ether)

pyridine (or ether)

*�� =


Carbon-Carbon Bond Forming Reactions and New Takes on Old Reactions

(C) Pavel Sedach Learnfaster.ca

45


Carbon-Carbon Bond Forming through Substitution 46


To deprotonate a carbonyl alpha hydrogen or an alkyne, your choices of base are either alkali metals (�� etc.) or an ��

derivative � an �� can work.

��

��

��

��


Grignard Reagents W2016Q17 47


Grignard reagents are prepared from the reaction of alkyl or aryl halides with Magnesium (Mg) metal in the presence of dry ether or THF.

Mg +

��

��+

�� HOMgBr+

Formation of a Grignard Reaction of a Grignard

Grignards can react with any of the carbonyls given below.The reactivity trend of the carbonyls is shown but, for the midterm, the trend is relatively unimportant:

��

Grignard reagents can also add to nitriles (� � �) and can do standard �� reactions.

Amide Ester Carboxylic Acid Ketone Aldehyde Acid Anhydride Acyl Chloride

< < < < < <


Simple reactions of Grignard Reagents 48


To generate the Grignard, we react an alkyl halide in an aprotic solvent (ideally under nitrogen)


Reactions of Grignard Reagents 49


Grignard reagents do not add to carboxylic acids - they simply deprotonate them. Grignards behave as a strong base and are quenched by protic solvents.

�� all quench/protonate Grignards.

1. ��2. ��

1. ��


��H

Reaction of Epoxide with a Nucleophile under Acidic Conditions: �� Like

Halohydration of Alkenes � similar to Epoxide Ring Opening: 50


*The wedge and hash put emphasis on the anti-addition of OH to Br

��

major minor��

OR

major minor

� � ��

The most stable carbocation like center is attacked by a nucleophile. This is �� like� reactivity.The behavior of this reaction flips in basic conditions:

major��

Under basic conditions, the least substituted C is attacked via �� like� reactivity. The cyanide above can be replaced with a hydroxide or �� or even a Grignard reagent.

Reaction of Epoxide with a Nucleophile under Basic Conditions: �� Like


Review 51


��/��

NaOEt/HOEt

1. ��2. ��


1) BBN-9 2) N��

Hydration of Alkynes 52


��

��

(or ��)


Ozonolysis (�� ) of Alkenes and Alkynes W2016-Part7 53


Typical [R] are �� or DMS (dimethyl sulfide)

Typical [O] is ��

Other [O] and [R] may be used

cis or trans does not matter - they produce the same result

Consideration: Ozonolysis of alkenes can be used to open up rings.

��

��

[O] or [R]+

[O] or [R]

��

[O]

��

[R]

carbon-carbon bonds are difficult to break so oxidative conditions don�t matter with internal alkenes (4 R groups)

��+

+

+

*some structure determination questions play a trick �they state that formaldehyde is produced instead of ��:

vs. ��


Ozonolysis of Alkynes 54


Alkynes are more reactive � they always produce carboxylic acids

[O] or [R]

[O] or [R]

[O] or [R]

��

2 ��

+

��

��

��

+


Oxidation [O] and Reduction [R] 55


Oxidation is the addition of oxygen. Reduction is the addition of hydrogen (removal of oxygen)

NO DICE!

1° alcohols

2° alcohols

3° alcoholsPCC or PDC or

most [O]

PCC or PDC

PCC or PDC or most [O]

*Depending on pH, temperature, most [O] may be used to partially oxidize in the same manner as PCC/PDC. This is not part of Chem 353 so if you don�t see PCC/PDC, assume full oxidation.

�� or�� or

�� or�� (Jones Reagent)

most [O] =

most [O]

most [O]


Reduction of Carbonyls using Hydrides 56


Hydrides are a source of �� (anionic hydrogen), a nucleophile

��

Which is more reactive, �� or ��?*All of these reaction involve a workup with ��

��

Atom Electronegativity

Al 1.5

B 2.0

H 2.1��

+

��

��

��

��

��

��

��

��

No Reaction

No Reaction


Diels Alder Cyclizations 57


The diene is more reactive with electron donating groups whereas the dienophile is more reactive with electron donating groups:

heat

heat

heat+

+

+


Diels Alder Cyclization and s-cis vs. s-trans reactivity 58


heat

+

+

heat


Addition of Halides to Conjugated Systems 59


conjugate or 1,4-addition

cold

��

heat

��

direct or 1,2-addition

��

��/uv or ��/heat/peroxides

radical substitution of most stable position

some radical substitution, some addition


��

Forming Cyclopropanes: The Simmons Smith Reaction 60


It is not necessary to know mechanisms involving metals, e.g. Hg , Mg, Zn.

��, Zn and/or Cu

Carbenes:

Both Simmonds smith and Carbene reactions preserve cis/trans stereochemistry.

Why does �� react with a base a relatively weak base like a hydroxide?

Why did we choose to use potassium tert-butoxide?

��


Spectroscopy in Brief 61


Key Functional Groups for IR

◦ �� or �� alcohols (3300 �� broad),

◦ �� amines and amides (3500 ��, doublet if ��, singlet if ��)

◦ � � � carbonyl (1700 ��, in Chem 353, very precise � exact maximum position tells you exact functional group!)

Fingerprint Region <1300 ��

Nitriles and ethers also have very indicative peaks but typically aren�t included on the test. We don�t learn nitrile reactions but they can be reduced to amines just like carboxylic acids can be reduced to alcohols.

Number of NMR peaks = Number of types of Hs

Key Functional Groups for 1H NMRCarboxylic acids (10-12 ppm), aldehydes (9-10), ppm. The rest are typical a certain distance from alkyl halides so you say alkanes from 0 to 5. If there�s 1 proton, it�s a CH, 2 protons is a ��, 3 protons is a ��, 6 protons is two identical ��s � so they�re usually off of one carbon.

The same functional groups above are found in 13C NMR and you are provided the shift tables.


Mechanistic Practice W2013-Part5 62


Grignard Reaction with an Ester

Base Catalyzed Fischer Esterification (identical but the reverse of Base Catalyzed Saponification)

ether

NaOEt/HOEt


Mechanistic Practice W2016-Part5 63



Mechanistic Practice W2015Q24 64



Review Slides

(C) Pavel Sedach Learnfaster.ca

65


Rapid Assignment of Stereogenic Centers 66


1. Least important group points into page? Assign as normal.2. Least important group points out of page? Assign as normal then flip (only 2 options, R or S) � VERY useful for Fischer Projections!3. Least important group in the plane of the page? Switch the least important group with whatever is going into page, assign then flip.4. For Newman projections or additions, attempt to see which way the least important groups is pointing and assign relative to that. If you get good enough at visualization you�ll begin to handle all structures this way.Try assigning all 7 in under 1.5 minutes:

Left to right,topto bottomR, S, S, racemic, R, S, R


Oxidation Number 67


Formal charge is the number of extra electrons present on an atom. Oxidation number is how those electrons are used by the atom � does the atom keep all the charge or is there a large molecular dipole towards a more electronegative atom?

*The subscripts on the atoms simply indicate which oxygen or carbon is being discussed

Formal Charge Oxidation Number

��

��

�� it gains one e- from �� and one from the -ve charge


Review 68


Arrange the following in order of bond strength:

i

ii

iv

iii

Arrange in order of bond length :

Bond length is the opposite of bond strength, therefore iii>iv>v>ii>i

v

i. sp to spii. sp2 to sp2iii. sp3 to sp3iv. sp to sp3v. sp2 to sp2 BUT it is conjugated! � this lowers bond strength as the

double bond is effectively now a 1.5 bond

Therefore i>ii>v>iv>iii


Nomenclature 69


E/Z and R/S mandatory!

Ortho (1,2), Meta (1,3), Para (1,4) Benzene(s)

O (ortho) M (meta) P (para)

Know some special branches isopropyl, tert-butyl


Resonance 70


Resonance is the movement of electrons through � �� bonds (any bond above a single bond).Resonance essentially distributes electron density or charge across the whole molecule rather than keeping it on one atom - thiskeeps things stable.

Cannot have resonance: Can have resonance:

Has major and minor structures

The ideal resonance structure (1) does not violate the octet rule, (2) minimizes formal charge (as few +s and -s as possible), and, if formal charge is unavoidable, places the � (minus) formal charge on the most electronegative atom.

Cannot have resonance:

Has major and minor structures

Can have resonance:

major minor

0 � bonds and therefore NO resonance!


Importance of Resonance Contributors 71


A Protonated carboxylic acid does not have useful resonance

Has more than 1 major resonance structure

� Has a resonance hybrid

� Distributes negative charge better!

Deprotonated carboxylic acid has two identical major resonance contributors


Special Names 72


3 Carbon Branch (Propyl):

5-propyldecane

5-n-propyldecane

*n is just a generic term for a branch that implies the branch is a straight chain

5-isopropyldecane

4 Carbon Branch (Butyl):

5-n-butyldecane

5-sec-butyldecane

5-tert-butyldecane

5-isobutyldecane


Ring Strain 73


Strain Energy (kcal/mol)

Stability

3 4 5 6 7 8 9

Members in the Ring

0

5

10

15

20

25

30

10 11


Index of Hydrogen Deficiency 74


Index of Hydrogen Deficiency = Degrees of Unsaturation = Number of �� Molecules Missing

��

��

��

��

saturated

��

��

��

IHD = 1

IHD = 2

IHD = 0

IHD = 1

IHD = 2

IHD Indicates the number of Rings or Pi Bonds in a structure. Very important for analysis!


Index of hydrogen deficiency 75


What is the IHD of the following compound?

Manual method:

How many rings or � bonds does �� have?��

��

�� bonds or rings

Draw a possible structure for ��

��

�

��

��

��

Saturated Hs Actual Hs Halogens Nitrogens

The structure above has � � bonds or rings though many, many other answers are possible!

��

��

So IHD = 6

1 ring + � � bonds


72 g C 12 g H 16 g S�12 �1 �326 12 0.5�0.5 �0.5 �0.512 24 1

Elemental Analysis 76


A compound is found to be 16 % S, 72 % C, with the remaining mass H. What is a possible molecular formula for the compound?

��

What is the IHD of the above compound?

��

�

��

��

Saturated Hs Actual Hs Halogens Nitrogens

divide by molecular weight

divide by smallest number

Hence the formula of the compound is ��


Combustion analysis 77


A compound produces 0.1055 grams of �� and 0.2577 grams of �� through combustion analysis. If its molecular weight is 56.12 g/mol, what is its molecular formula?

0.2577 g �� 0.1055 g �� 18.02 g/mol��

��

� ��

� �

� ��

��

1 mol C 2 mol H

divide by molecular weight

divide by smallest number

The empirical formula is ��. This is 14.03 g/mol. 56.12 g/mol� 14.03 g/mol = 4

The molecular formula is ��

Find quantity of atoms of each element


Review 78


SDBSWeb : http://sdbs.db.aist.go.jp (National Institute of Advanced Industrial Science and Technology, October 28, 2015)

4000 3000 2000 10001500 500

4000 3000 2000 10001500 500

��IHD=4, likely

��IHD=0, likely

Notice the many new peaks that appear when you have an aromatic


Review 79



4000 3000 2000 10001500 500

4000 3000 2000 10001500 500

aniline

N-methylaniline

When there is an ��there are typically two peaks in the amine region

When there is an �� there is only one peak


Review 80



4000 3000 2000 10001500 500

4000 3000 2000 10001500 500

benzoic acid

benzamide

1640

1706

Two distinct peaks are present signifying an ��

Sometimes, the hydroxide OH is so broad it blends into other peaks

In Chem 351 exam, C=O stretches VERYPRECISELY match the functional group!


Intermolecular Forces 81


London Dispersion Forces

(Van Der Waals Forces)

Dipole-Dipole Bonding

Compound � State at 25° C - # of Electrons

�� - GAS � 34 electrons

�� - LIQUID � 70 electrons

�� - SOLID � 106 electrons

*The more electrons, the greater the possible LDF

Hydrogen Bonding(FON Elements)

HF

The net dipole in molecules with uneven electron distribution causes them to stack end to end like magnets!

((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((VVVVVVVVVVVVVVVVVVVVVVaaaaaaaaaaaaaaaaaaaaaaannn

Hydrogen Bonding is the Strongest Intermolecular Force and keeps molecules together. It can be nearly as strong as a covalent bond and it is what keeps DNA together


Effects of Intermolecular Forces on Boiling Point 82


What are the intermolecular forces present in pure solutions of the following compounds?

Rank the following according to boiling point (high to low)

��

��

i ii iii

LDF LDFweak D-D (dipole)

LDFD-D

H-Bonding

< < Boiling Point

LDF

*there is NO net dipole!

LDFD-D

H-Bonding

LDFD-D


Review 83


Rank the following according to boiling point (high to low)

i ii iii

��

��

��

The more linear the alkane, the better it stacks and has a stronger London Dispersion or induced dipole-induced dipole interaction

*LDF is also known as Van Der Waals and also known as induced dipole-induced dipole


Exceptions 84


Resonance causes some structures to behave not as they are drawn (e.g. to have a different effective hybridization)

What is the hybridization of N1 and O2?

N1 is sp3 and Tetrahedral butTrigonal pyramidal if we look at molecular shape

O2 is sp3 and Tetrahedral but bent <109.5 if we look at the molecular shape

Where are the lone pairs found in N1 and O2?On N1, the lone pair is in an sp3 orbital

In O2, the lone pairs are in sp3 and sp3

What is the hybridization of O1, N2 and O3?sp2, sp2 and sp2

Where are the lone pairs found in O1, N2 and O3?In O1, one lone pair is in an sp2, the other made the � bond and is in a p orbitalIn N2, the lone pair is in a p orbitalIn O3, the lone pairs are in p and sp2.


Reduction of Carbonyls 85


Aldehydes and ketones can be reduced by sodium borohydride (��/ethanol) or lithium aluminum hydride (��/ether). An acid workup yields the final alcohol.

�� is far more reactive than �� (weaker Al-H bond vs. B-H)

��

��


Conformations of cyclohexane 86


The conformation of cyclohexane does NOT affect the line diagram and is not reflected in the line diagram

Chair Boat Chair

Ring-Flip Ring-Flip


Conformations of cyclohexane 87


Large groups like tBu or OtBu like to be equatorialQuestion: What is the relative rate of reactivity with NaOEt/EtOH?

��

��

i ii iii

d) Because tBu is a large group and prefers equatorial. In i, the Br shares the same alternating position as the tBu and therefore Br must be equatorial. In ii, the Br is opposite of what it is in i, therefore it must be axial. In structure iii, the Br has a highprobability of being axial or equatorial.


Types of Hydrogen/Carbon - Alkanes 88


3 types C

3 types H

2 types C2 types H

2 types C2 types H

4 types C4 types H

1 types C1 types H


Types of Hydrogen/Carbon - Alkenes 89


4 types C

4 types H

Symmetry

There are different types of symmetry � not just reflection over a plane!

5 types C3 types H

Electronic Environment

3 types C4 types H

3 types C3 types H

3 types C3 types H


Types of Hydrogen/Carbon - Cyclic 90


Because you cannot completely free rotate around single bonds in a ring, rings behave like double bonds � this leads to many types of hydrogen!

1 types C1 types H

3 types C4 types H

4 types C5 types H

4 types C6 types H


��NMR of Benzocaine 91


Spend less time on �� NMR. Strictly note the number of types of carbons and the specific functional group. �� is so rare(1% abundance only) that there is no information on coupling (splitting) or relative quantity (integral)

*IF number of carbons in spectra does NOT match molecular formula, look for symmetry!

Number of types of carbon is number of 7 � same as the number of peaks.

220 200 180 160 140 120 100 80 60 40 20 0

� � � ��

� � �

� � �aldehyde or ketone

carboxylic acid derivative

� � � � � �

Benzene Cs


��NMR of Benzocaine 92


1H NMR is your main tool! After flying through the other analyses (finding IHD, noting some possible functional groups/heteroatoms and number and types of carbons, we get to Multiply the integrals in the 1H NMR until they give you the total number of Hs in the entire spectra.

Number of types of hydrogen is number of 7 � same as the number of peaks.

10 9 8 7 6 5 4 3 2 1 0

��/��

� � �� / � � ��

H−C−NH−C−S

� � � � �H−C−C=OH−C−C=C

1112

� � � � �

alkyl ��

10 9 8 7 6 5 4 3 2 1 0


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