Pka alcoholes

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Chapter 17: Alcohols and Phenols
C O O H H

sp3

alcohol pKa~ 16-18

phenol (aromatic alcohol) pKa~ 10

Alcohols contain an OH group connected to a saturated carbon (sp3) Phenols contain an OH group connected to a carbon of a benzene ring

O H C C

H C C

O

chemistry dominated by the keto form
76

enol

keto

H

O

H

R

O

H

R

O

R

R

O

O

R'water

alcohol
R S H

ether
R S R

peroxide
R S S R'

thiols

thioether

disulfides

Alcohols are classified as primary (1°), secondary (2°) or tertiary (3°), which refers to the carbon bearing the hydroxy group

methanol
H C O H H H

primary
R C O H H H

secondary
R C O R H H

tertiary
R C O R R H
77

1° carbon

2° carbon

3° carbon

39

17.1:Nomenclature: 1. In general, alcohols are named in the same manner as alkanes; replace the -ane suffix for alkanes with an -ol for alcohols
CH 3CH 2CH 2CH 3 CH 3CH 2CH 2CH 2OH OH

butane

1-butanol

2-butanol

2. Number the carbon chain so that the hydroxyl group gets the lowest number 3. Number the substituents and write the name listing the substituents in alphabetical order. 4. For phenols,follow benzene nomenclature and use phenol as the parent name. The carbon bearing the -OH group gets number 1. 78

OH

OH OH NO2 NO2

2-methyl-2-pentanol

3-phenyl-2-butanol

3,4-dinitrophenol

Many alcohols are named using non-systematic nomenclature

OH

OH

H3C C OH H3C H3C

OH HO OH HO OH

benzyl alcohol (phenylmethanol)

allyl alcohol (2-propen-1-ol)

tert-butyl alcohol(2-methyl-2-propanol)

ethylene glycol (1,2-ethanediol)

glycerol (1,2,3-propanetriol)

79

40

17.2: Properties of alcohols and phenols: Hydrogen bonding: The structure around the oxygen atom of an alcohol or phenol is similar to that in water and is sp3 hybridized Alcohols and phenols have much higher boiling points than similar alkanes and alkyl halides
H2 O MW=18 bp= 100° C C6H6MW=78 bp= 80° C CH3CH2CH2CH3 MW=58 bp= -0° C C6H6OH MW=94 bp= 182° C CH3CH2CH2CH2Cl MW=92.5 bp= 77° C C6H6CH3 MW=92 bp= 110° C CH3CH2CH2OH MW=74 bp= 116° C C6H6CH2OH MW=108 bp= 203° C

80

Alcohols and phenols, like water, can form hydrogen bonds: non covalent interaction between a hydrogen atom (δ+) involved in a polar covalent bond, with the lone pair of a heteroatom (usually O or N), whichis also involved in a polar covalent bond (δ-)
O H !- !+ O H
H O H O H O H H O H H O H

N H !- !+ N H

C O !+ !C O

C O

H

H O H O H O H

H

H O H

H O H

H
H

R O!

!

!

H

H O! R
!

R O!

H

!

H O! R
!

R O!

!

H

O! R

H O H

H

H O H

Hydrogen-bonds are broken when the alcohol reaches its bp, 81 which requires additional energy

41 17.3: Properties of alcohols and phenols: acidity and basicity: Like water, alcohols are weak Brønsted bases and weak Brønsted acids. The nature of the R group can significantly influence the basicity or acidity
H R O H

+

H X

R

O

H

+ :X-

oxonium ion
R O H

+

H O H

H R O

+

H

O

H

alkoxide ion CH3CH2CH(OH)CH3 MW = 74 bp = 99° C pKa ~ 17 (CH3)C-OH MW =74 bp = 82° C pKa ~ 18

CH3OH MW = 32 bp= 65° C pKa ~ 15.5

CH3CH2CH2CH2OH MW = 74 bp = 116° C pKa ~ 16

The steric environment around the oxygen atom can influence the physical properties of an alcohol

82

Solvation: upon acid dissociation the alkoxide ion is stabilized by solvation through hydrogen bonding between water and the negatively charge oxygen. The steric environment aroundthe negatively charge oxygen influences the solvation effect
R O H

+

H O H

H R O

+

H

O

H

H

O H O H H H H O H O H H H H

O H H C H C O H C C H H H H H H O H O H

H H C O H H H O

Acidity: methanol > 1° alcohol > 2° alcohol > 3° alcohol Reflects the ability water to stabilized the resulting alkoxide 83 though solvation

42

Electronic factors that influence...
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