Hemiacetal and Acetal Functional

Groups Hemiacetal: alcohol and ether on same carbon Acetal: Two ethers on same carbon

C

H2C CH3

OH

O

H3C

H

CH3

C

H3C OH

OH3C

H3C

CCH3

O CH3O

H3C

Mechanism

Must be acid-catalyzed. Adding H+ to carbonyl makes it more

reactive with weak nucleophile, ROH. Hemiacetal forms first, then acid-catalyzed

loss of water, then addition of second molecule of ROH forms acetal.

All steps are reversible. =>

Hemiacetal Synthesis: aldehyde + alcohol

Alcohol + aldehyde --> hemiacetal Ethanal + methanol Alcohol oxygen becomes an ether Carbon double bond oxygen becomes an alcohol

CH

H3C O H3C O+H

CCH3C

H

CO

CH3C

OH

Mechanism for Hemiacetal

OH

+

+OH

H+

O

=>

HO OCH3+

HO OCH3H

HOCH3

OH

+ HOCH3H2OCH3++

Aldehyde and Ketone Reactions

Synthesis of Acetal Functional Group Acetal: alcohol plus hemiacetal (ether synthesis) Acetal: Two ethers on same carbon

CH3C

O

OCH3

H

H

HO

H

H3C O+H

C

H3C

OH3C

O

CH3

H

=>

Aldehyde and Ketone Reactions

Mechanism Hemiacetal to Acetal

+

OCH3HO OCH3

H+

H+

HO OCH3

HOH+

=>

OCH3CH3OOCH3CH3O

H

+

OCH3+

HOCH3

HOCH3

Cyclic Acetals

Addition of a diol produces a cyclic acetal. Sugars commonly exist as acetals or

hemiacetals.

O

CH2 CH2

HO OH+

O OCH2 CH2

=>