Transcript Hydrogen bonding

Organic chemistry for medicine and biology
students
Chem 2311
Chapter 7
Alcohols, Phenols and Thiols
By Prof. Dr.
Adel M. Awadallah
Islamic University of Gaza
Nomencalture of alcohols
Use the end ol
Examples
CH3OH
CH3CH2OH
CH3CH2CH2OH
Methanol
Methyl alcohol
Ethanol
ethyl alcohol
1-propanol
propyl alcohol
CH3CHCH3
OH
2-propanol
isopropyl alcohol
CH3
H3C
CH2OH
H3C
2-methyl-1-propanol
isobutyl alcohol
H3C
C
OH
CH2 = CH - CH2OH
CH3
2-methyl-2-propanol
tert-butyl alcohol
2-propen-1-ol
allyl alcohol
OH
OH
HO
cyclopentanol
OH
cis-1,2-cyclopentandiol
2-phenylethanol
OH
OH
H3C
C
C
CH2OH
C
H2
NO2
3-pentyn-1-ol
COOH
p-nitrophenol
phenol
CHO
OH
OH
o-hydroxybenzoic acid
salicylic acid
m-hydroxybenzaldehyde
Hydrogen bonding in alcohols and phenols
• Alcohols and phenols form hydrogen bonds, and hence
they have relatively high boiling points. This also makes
the lower alcohols miscible with water. As the R group
becomes larger, the solubility of alcohols in water
decreases dramatically.
• Hydrogen bonding
• Hydrogen bonding occurs between molecules where you
have a hydrogen atom attached to one of the very
electronegative elements - fluorine, oxygen or nitrogen.
Acidity of Alcohols
Acids are proton donors.
The acidity increases as the negative charge at the OH decreases
(delocalized):
a)
phenols are more acidic than Alcohols due to resonance effect
(delocalization of the negative charge)
b)
Nitrophenols are more acidic than phenols due to resonance
and inductive effect (The partial neutralization of the negative
charge by a nearby positive charge).
c) Electron withdrawing groups attached to alcohols increase
the acidity of alcohols due to inductive effect.
Cl
Cl
C
Cl
CH2 - OH
>
Cl
H C
CH2 - OH
Cl
>
Cl
H C
CH2 - OH
H
>
H
H C
CH2 - OH
H
d) Remember; Thiols are more acidic than alcohols because the
sulfur atom is larger than oxygen, and hence carries the
negative charge easily.
R - S -H
Ar - S -H
>
>
R-O-H
Ar - O - H
Alcohols do not react with NaOH, but thiols react with NaOH.
ROH + NaOH
RSH + NaOH
====== No reaction
====== R - S - Na+ + H2O
Alcohols , however, react with the stronger bases Na or NaH
ROH = Na (or NaH) ======= R - O- Na+
+
H2
Preparation of ethanol
Ethanol is manufactured by reacting ethene with •
steam. The catalyst used is solid silicon dioxide
coated with phosphoric(V) acid. The reaction is
reversible.
•
Only 5% of the ethene is converted into ethanol •
at each pass through the reactor. By removing
the ethanol from the equilibrium mixture and
recycling the ethene, it is possible to achieve an
overall 95% conversion.
Making ethanol by fermentation
This method only applies to ethanol. You can't make any
other alcohol this way.
• Yeast is killed by ethanol concentrations in excess of
about 15%, and that limits the purity of the ethanol that
can be produced. The ethanol is separated from the
mixture by fractional distillation to give 96% pure ethanol.
• For theoretical reasons, it is impossible to remove the
last 4% of water by fractional distillation.
The manufacture of other alcohols from alkenes
Take care of Markovnikov and anti Markovnikov
additions
Reactions of Alcohols
Acidic dehydration produces alkenes with the more substituted double
bond
(OH- is a bad leaving group, but H2O is a good leaving group, so the
reaction starts by protonation of the OH group
H+
H3C
CH2 - OH
180 oC
CH2 = CH2
Note: this reaction gives diethyl ether when heated only to 140 oC
Mechanism: (E2)
Dehydration of tertiary butyl alcohol
Examples
OH
H+
CH3 - CH - CH2 - CH3
CH3 - CH = CH CH3
heat
CH3
OH
+
CH2 = CH - CH2 - CH3
major
H+
CH3
heat
major
minor
CH2
+
minor
Reaction of Alcohols with Hydrogen Halides
The general reaction looks like this:
A tertiary alcohol reacts if it is shaken with concentrated
hydrochloric acid at room temperature . This reaction occurs by SN1
mechanism, so the reaction rate is almost the same with HCl, HBr or
HI, since the addition of the halide nucleophile occurs in the second
fast step.
Primary Halides:
The reaction is very slow with primary chlorides, and may occur by
heating them with ZnCl2 for several hours.
Since this reaction occurs by SN2 mechanism, the order of reactivity is:
I > Br > Cl
• Reaction with phosphorus(III) chloride, PCl3
• Alcohols react with liquid phosphorus(III) chloride (also called
phosphorus trichloride) to make chloroalkanes.
Reacting alcohols with sulphur dichloride oxide (thionyl chloride)
• The reaction
• Sulphur dichloride oxide (thionyl chloride) has the formula SOCl2.
• The two other products of the reaction (sulphur dioxide and HCl) are both
gases. That means that they separate themselves from the reaction mixture.
• Hydrogen halides, phosphorous halides or thionyl
halides cannot replace the hydroxyl group of phenols
by halogens
HX
No reaction
AR - OH
PX3
SOCl2
Oxidation of Alcohols
Primary alcohols are oxidized to aldehydes using pyridinium
chlorochromate (PCC).
Oxidation by KMnO4, K2Cr2O7 or CrO3 dissolved in sulfuric acid
(Jones’ reagent) gives the corresponding carboxylic acids).
Oxidation of secondary alcohols (gives ketones)
Oxidation of tertiary alcohols (don’t occur)
Polyhydroxy compounds
CH2 - CH2
CH2 - CH - CH2
OH
OH
OH
ethylene glycol
(1,2-ethandiol)
b. p. 198 oC
OH
OH
OH OH
Glycerol (glycerine)
(1,2,3-propantriol)
b. p. 290 oC
OH
CH2 - CH - CH - CH - CH - CH2
OH
OH OH OH
OH OH
sorbitol
(1,2,3,4,5,6-hexanhexaol)
m. p. 110 - 112 oC
OH
OH
OH
OH
OH
catechol
Resorcinol
Hydroquinone
OH
pyrogallol
Electrophilic aromatic Substitution in Phenols
The hydroxyl group is an activating group. It is o, p directing
group
OH
OH
Br
Br
OH
dilute HNO3
3 Br2
EtCl
AlCl3
Br
OH
Et
NO2
Oxidation of Phenols
O
OH
Na2Cr2O7
H2SO4, 30 oC
OH
hydroquinone
O
1,4-benzoquinone
Antioxidant Phenols
Phenols function as antioxidants. They destroy peroxy free radicals (ROO.)
OH
OH
OH
C(CH3)3
C(CH3)3
(CH3)3C
C(CH3)3
OMe
OMe
CH3
BHA
Butylted hydroxy anisol
CH3
O
H3C
BHT
Butylated hydroxy toluene
CH3
CH3
CH3
HO
CH3
CH3
Vitamine E ( -tocopherol)
Natural phenolic antioxidant
CH3
Thiols
Nomenclature
CH3CH2CH2CH2SH
CH3SH
SH
1-butanrthiol
(n-butyl mercaptan)
Methanethiol
(Methyl mercaptan)
thiophenol
(phenyl mercaptan)
O
H3C
SH
thiolacetic acid
Preparation
R – X + SH- == R – SH
H3C
S
CH3
Ethyl sulfide
+ X-
Reaction of thiols with NaOH
RSH + NaOH = RS- Na+
Dislfides
+ H 2O
oxidation
2 RSH
RS - SR
reduction
thiol
disulfide
S
S
diallyl disulfide
responsible for the odor of garlic (plant family allium)