Carbonyls

Finishing Chapter 21 Chapter 16 - Intro to Carbonyls Chapter 15 - Highlights Grignard and Gilman reagents

Lecture 11 Sessler 310N

Recommended Problems CH’s 20; 21 20.4, 20.6, 20.7-20.10, 20.16, 20.17, 20.19-20.32, 20.43, 20.46-20.48 21.2, 21.4, 21.5, 21.7-21.9, 21.11, 21.14-21.22, 21.24-21.26, 21.30-21.43

Exam I will cover spectroscopy (MS, IR, NMR, UV-Vis) and Aromaticity (Chapts. 12-14; part of 23; 20 and 21.1-21.2) NOTE: NO OFFICE HOURS TOMORROW (SORRY). However, … There is a review session Tonight (5 pm; Wel 2.122)

Di- and P olys olysubs ubstit itut ution ion z

z

Existing groups on a benzene ring influence further  substitution in both orientation and rate Orientation:  – certain substituents substituents direct preferentially to ortho & para positions; others direct preferentially to meta positions  – substituents are are classified as either 

ortho- para directing or  meta directing

Di- and P olys olysubs ubstit itut ution ion z

Rate:  –  certain substituents cause the rate of a second substitution to be greater than that for benzene itself; others cause the rate to be lower   –  substituents substituents are classified as • •

activating toward further substitution, or deactivating preventing or slowing further substitution

Di- and Polysubstitution z

-OCH3 is ortho- para directing and activating.

THE MECHANISTIC EXPLANATION ACCOUNTING FOR THIS OBSERVATION WAS PRESENTED IN CLASS THURS. z

OCH3

OCH3 B r 2

OCH3

B r  +

+

CH 3 CO 2 H B r  anisole o -bromo-

p -bromo-

anisole (4%)

anisole (96%)

HBr 

Di- and Polysubstitution -NO2 is meta directing and deactivating!

z

NO 2 + HNO

H 2 SO 4 3

NO 2

NO 2

NO 2 NO 2

Nitrobenzene

+

+ NO 2 m-Dinitrobenzene (93%)

o-Dinitrobenzene

NO 2 p-Dinitrobenzene

Less than 7% combined

THE MECHANISTIC EXPLANATION ACCOUNTING FOR THIS OBSERVATION WAS PRESENTED IN CLASS THURS. z

Methyl Group CH3

Toluene undergoes nitration 1000 times faster than  benzene. A methyl group is an activating substituent.

THE MECHANISTIC EXPLANATION ACCOUNTING FOR THIS OBSERVATION WAS PRESENTED IN CLASS THURS. z

Relative rates of  Nitration OH

1000

Cl

H

1.0

0.033 Reactivity

NO 2

6x10-8

Halogen Substituents Cl

Chlorobenzene undergoes nitration ca. 30 times more slowly than  benzene . Halogens are deactivating substituents. But, they are o, p  p directors!

How can this be???

Back to…

Theory of Directing Effects • So…what’s going on here???? • The rate of EAS is limited by the slowest step in the mechanism…duh • For EAS, the rate-limiting step is attack of E+ on the aromatic ring to form a resonance-stabilized cation intermediate • The more stable this cation intermediate, the faster the ratelimiting step and the faster the overall reaction • In the case of halogen, competing effects are at work!

Use this Table to Practice and Review

Real Fast Pretty fast Kinda slow Pretty slow Slow Real Slow

To Review: Effect on Regioselectivity • Ortho- para directors direct an incoming electrophile to positions ortho and/or para to themselves. •  Meta directors direct an incoming electrophile to  positions meta to themselves. •  All meta directors are deactivating •  All ortho- para directors are activating except  halogen

Polysubstitution--General Rules z

z

z

Where more than one substituent exists on a benzene ring, look to see if one is significantly more activating than the other. If this is the case and open ortho and/or  para site(s) are available, the incoming electrophile will substitute at these latter sites. Groups such as -OR and -NR 2 dominate over more weakly activating groups, such as alkyl groups, and halogens If groups are similarly activating or deactivating and they are not oriented to direct towards the same position (s), such as in the case of 1,3-dinitrobenzene, horrible mixtures can result. Plan your synthesis to avoid mixtures!

The wrong way…. CO2H

CH3

NO2

HNO3

Na2Cr2O7

CO2H

CO2H

+ H2SO4

H2SO4 NO2

NO2

NO2 NO2

NO2

The right way… NO2

NO2 Na Cr O 2 2 7

HNO3

H2SO4

H2SO4 NO2

CO2H

CH3

CH3

NO2

NO2

Explain this with mechanism(s)! Chemistry 310N 

And one last “minimum expectation” review problem:

ortho Nitration of Toluene H

CH3 NO2 +

H

H H

Be sure you can write the remaining resonance structures and then do the same for meta and for  para  para attack.

H Practice going from your list of structures to a coherent argument about which products will be formed in preference and why.

End of Material That will be Covered on Exam I

 Note how Sect. 21.3, Nucleophilic Aromatic Substitution, is not going to be on the Exam or, in fact, covered in the course.

Chemistry 310N 

Lecture 10 Cont. Carbonyl Chemistry I

Suggested Homework Problems (for this lecture) 15.13, 15.14a 16.4, 16.15, 16.16, 16.19-16.23

-

O

O

C

C

+

 The Carbonyl Group z

Over the course of several chapters we will study the physical and chemical properties of classes of compounds containing the carbonyl group, C=O  – aldehydes and ketones (Chapter 16 (+15.1))  – carboxylic acids (Chapter 17)  – acid halides, acid anhydrides, esters, amides (Chapter 18)  – enolate anions (Chapter 19)

Resonance Description of  Carbonyl Group ••

O• • C

•• –

• •O • • C +

nucleophiles attack carbon; electrophiles attack oxygen

 The Carbonyl Group z

The carbonyl group consists of   – one sigma bond formed by the overlap of sp2 hybrid orbitals, and  – one pi bond formed by the overlap of parallel 2p orbitals π

C

σ

O

Structure of Formaldehyde

The molecule is planar   bond angles: close to 120° C=O bond distance: 1.22 Å

Bonding in Formaldehyde

Carbon and oxygen are sp2 hybridized

Bonding in Formaldehyde

 The p orbitals on carbon and oxygen overlap to form a π bond

Nomenclature-Aldehydes z

IUPAC names: select as the parent alkane the longest chain of carbon atoms that contains the carbonyl group..subtract e and add al  – because the carbonyl group of the aldehyde must be on carbon 1, there is no need to give it a number 

z

For unsaturated aldehydes, show the presence of the C=C by changing -an- to -en – the location of the suffix determines the numbering  pattern

Nomenclature-Aldehydes 5

4

3

2

CH 3

O

1

O

4

CH 3 CH 2 CH 2 CH 2 CH

3 2 1 CH 3 CHCH 2 CH

Pentane

3-Methylbutanal

Pentanal

O

O 3

2

1

CH 2 =CHCH 2-Propenal (Acrolein)

5

7 8

6

3 4

1 2

H

(2E)-3,7-Dimethyl-2,6-octadienal (Geranial)

Nomenclature of Aldehydes O

O H

H

4,4-dimethylpentanal O

5-hexenal O

HCCHCH 2-phenylpropanedial

Nomenclature of Aldehydes O when named as a substituent this is a formyl group

C

H

when named as a suffix it is carbaldehyde carboxaldehyde

Nomenclature-Aldehydes z

For cyclic molecules in which the -CHO group is attached to the ring, the name is derived by adding the suffix -carbaldehyde to the name of the ring 1

CHO

CH 3 2 CH 3 2,2-Dimethylcyclohexanecarbaldehyde

CHO 1 2 3

2-Cyclopentenecarbaldehyde

Many aldehydes and ketones occur naturally O

2-heptanone (component of alarm pheromone of bees)

O

trans-2-hexenal (alarm pheromone of myrmicine ant)

Structure of Ketones z

The functional group of a ketone is a carbonyl  group bonded to two carbon atoms

O CH3 -C-CH 3 Propanone (Acetone)

O

Cyclohexanone

 Trivial Nomenclature of Ketones O

O

CH3CH2CCH2CH2CH3

CH2CCH2CH3

ethyl propyl ketone benzyl ethyl ketone

O divinyl ketone H2C

CHC CH

CH2

IUPAC Nomenclature-Ketones z

IUPAC names: – select as the parent alkane the longest chain that contains the carbonyl group, – number to give C=O the smaller number and then subtract e and add one 7

O

O 1

2

3 4

CH3

5

6

CH 3 CCH 3

CH 3 CH2 CCH 2 CHCH 3

Propanone (Acetone)

5-Methyl-3-hexanone

1

6 5

O 4

2 3

Bicyclo[2.2.1]-2heptanone

IUPAC Nomenclature-Ketones O 1

2

3

4

5

CCH 2 CH 2 CH 2 CH 3

1-Phenyl-1-pentanone z

Even the IUPAC system retains the common names acetone, acetophenone , and benzophenone

O CH 3 CCH 3 Acetone

O

O

CCH 3

C-

Acetophenone

Benzophenone

IUPAC Nomenclature of Ketones O O CH3CH2CCH2CH2CH3 3-hexanone

CH3CHCH2CCH3 CH3 4-methyl-2-pentanone O

O

H

CH3CH2CCH2CH2CH3 ethyl propyl ketone

O

4-oxohexanal

Order of Precedence (Pecking order) z

For compounds that contain more than one functional group indicated by a suffix Functional Group      e      n      e        d      e      c      e      r        P

Suffix if Higher  Prefix if Lower  in Precedence in Precedence

-CO 2 H

-oic acid

-CHO

-al

oxo-

-one

oxo-

-ol -amine -thiol

hydroxyaminomercapto-

C=O -OH -NH 2 -SH

Synthesis of Aldehydes and Ketones A number of  reactions already studied provide efficient synthetic routes to aldehydes and ketones. Review these after the exam since they are expected knowledge.

• from alkenes • by ozonolysis

• from alkynes • by hydration (via enol)

• from arenes • via Friedel-Crafts acylation

• from alcohols by oxidation

Reactions of Carbonyls z

z

We will cover quite a number, including early on ones where carbon acts as an electrophile and the oxygen acts as a nucleophile or, more commonly, base. But first, oxidation chemistry of aldehydes: O

O

[oxidant] H

H2O

OH

oxidant = Cr(VI), Mn(VII), Ag(I), etc.

We had this last semester. Review the material and mech!