Pka alcoholes
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
4117.3: 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...
Regístrate para leer el documento completo.