11

Some Important Reactions of Some Important Reactions of AlcoholsAlcohols

1. Dehydration = loss of H1. Dehydration = loss of H22O to O to form form an alkenean alkene

2. Oxidation = loss of H to form 2. Oxidation = loss of H to form a a C=O compoundC=O compound

22

Dehydration ExamplesDehydration Examples

CH3CHCH3 CH2=CHCH3 + H2OOH2-propanol --> propene + water2-propanol --> propene + water

general pattern:general pattern:

alcohol --> alkene + wateralcohol --> alkene + water

33

HH

OH

HAnother exampleAnother example

H2O +

cyclopentanol --> cyclopentanol --> cyclopentene + watercyclopentene + water

44How does reaction occur?

* Loss of water from alcohol alone has high activation energy = alcohols dehydrate very, very, very slowly by themselves

* speed up reaction by lowering Eact with catalyst

55

Catalyst for dehydration of alcohol is usually acid =

H+ from H2SO4 (sulfuric acid) or

enzyme in living organism.

66Main thing to focus on:

C-C C-C

OH

H OH H

HH+

+

lose H+& H2OC=C

alkene

C=C forms C=C forms between C between C bonded to OH bonded to OH and adjacent C and adjacent C bonded to H lost bonded to H lost as Has H++

77

When the C-O bonds breaks to lose H2O, an adjacent C-H bond breaks to lose H+.

The result is formation of the The result is formation of the new pi (new pi () bond to make the ) bond to make the alkene.alkene.Catalyst helps OH leave as HCatalyst helps OH leave as H22O O from alcohol.from alcohol.

88When there is more than one kind of H on the C next to C-OH, more than one alkene can form:

CH3CHCH2CH3 H2O +OH

H+

heat

CH2=CHCH2CH3 + CH3CH=CHCH3

99Oxidation Reaction Examples

CH3CH2-CH-CH3

OH[O]

CH3CH2-C-CH3

O

2° alcohol2° alcohol

ketoneketone

oxidizing oxidizing agentagent

1010General reaction:

R-CH-R’ R-C-R’

OH O[O]

2° ROH + [O] ---> ketone2° ROH + [O] ---> ketone

1111Oxidizing agents remove H and e- from the grouping

H-C-O-H that makes up the alcohol group to form C=O.

Many reagents can do this: common ones are Cr or Mn compounds. Even household bleach (NaOCl) can be [O].

1212Primary alcohols can be oxidized in 2 stages:

R-CH2-OH R-C-H

O[O]

aldehydealdehyde

R-C-H

O[O]

R-C-OHO

carboxylic acidcarboxylic acid

1313Aldehydes are easier to oxidize than 1° alcohols, so it is sometimes difficult to stop the reaction at the aldehyde stage.

Special reagents have been developed to make it possible to get aldehydes by oxidizing a 1° alcohol.

1414Tertiary alcohols are not oxidized because there is no H on the C bonded to OH:

R-C-O-H is 3° alcohol

R

RCH3-C-O-H

CH3

CH3

no H on Cno H on C

1515Phenols are benzene compounds with an OH group attached to a sp2 C in ring:

OH

““phenol” salicylic acidphenol” salicylic acid

OH

CO2H

1616Phenols & Alcohols are not alike!

* both have C-O-H* both have C-O-H* both form H-bonds, * both form H-bonds, BUT:* alcohols are neutral * alcohols are neutral

compoundscompounds* phenols are acidic* phenols are acidic

1717ETHERS have the general formula

R-O-R’ R-O-R’ (has C-O-C bond)(has C-O-C bond)

*bond angles about 109°*tetrahedral e- pairs on O*C-O-C shape is angular or

bent

1818

CHCH33CHCH22-O-CH-O-CH22CHCH33

“diethyl ether” is symmetrical

CHCH33-O-CH-O-CH22CHCH22CHCH33

““methyl propyl ether” is methyl propyl ether” is unsymmetricalunsymmetrical

both Alkyl groups can be the same or different:

1919

non-IUPAC names for ethers = “common names”

name of alkyl group 1 name of alkyl group 1 (space) name of alkyl group 2 name of alkyl group 2 (space) etherether

see previous slide for examples

2020Ethers do not provide a H for a hydrogen bond, but can provide the electron pair of O.

So: ethers are H-bond acceptors,

they donate :O: to the H of another O-H or H-N

2121

R

R

O••

••

etherether

H-bond H-bond acceptoracceptor

H-OR’

alcoholalcohol

H-bond donorH-bond donor

2222Sulfur Analogs of Alcohols and Ethers

R-S-H thiol or mercaptan

R-S-R’ thioether

R-S-S-R’ disulfide

Simply be able to recognize the Simply be able to recognize the type from structure.type from structure.