Transcript Acidity of Alcohols

Organic chemistry A
Chapter 6
Alcohols and Ethers
By Prof. Dr.
Adel M. Awadallah
Islamic University of Gaza
Nomenclature of Organic Compounds
Alkanes
Alkenes
Alkynes
Alcohols
Ketones
Aldehydes
Acids
ane
ene
yne
ol
one
al
oic acid
Nomenclature of alkenes and Alkynes
1)
2)
3)
4)
The ending ene is used for alkenes and yne for alkynes
Select the longest chain that includes both carbons of the
multiple bond
Number the chain from the end nearest to the multiple bond
Indicate the position of the multiple.
Examples
1
1
CH3
H3C
2 3
1
Br
4 CH3
5
4-bromo-2-methyl-2-pentene
CH2
H3C
6 5 4 3 2
2-propyl-1-hexene
Isomers and common names of
simple alkenes
H
H
H
H
H
H
H
Cl
Ethene (ethylene)
H
H
H3C
H
vinyl chloride
Cl
H
H
CH2
1-butene
H
H3C
H
C
H2
H
Allyl chloride
Propene (propylene)
H3C
H
H
H
CH3
H2C
CH3
CH3
cis-2-butene
H3C
H
trans-2-butene
H3C
2-methyl-1-propene (isobutene)
Assigning Priority
• Alkenes and alkynes are considered to have
equal priority
• In a molecule with both a double and a triple
bond, whichever is closer to the end of the chain
determines the direction of numbering.
• In the case where each would have the same
position number, the double bond takes the
lower number.
• In the name, “ene” comes before “yne” because
of alphabetization.
7
6 5
4 3
2
1
7
4-heptyn-2-ene
7
6 5
4 3
2
4-hepten-2-yne
6 5
2-hepten-5-yne
4 3
not
2
1
5-heptyn-2-ene
1
Compounds containing more than one double
bond
CH3
1 2
3
2
3 4
4
5
1
2-methyl-1,3-butadiene
7
4 3 2
5
1
1,3,5-heptatriene
6
4
7
3
8
2
3
Br
5
2
6
1
3-methylcycopentene
5
6
4
CH3
1
1,3,5-cyclooctatriene
5-bromo-1,3-cycohexadiene
Cis-Trans (Z-E) Isomerism in Alkenes
If each end of the double bond is attached to two
different groups, then the compound exists in two
different forms called (Diastereomers; These are non
mirror image stereoisomers).
If the two groups are identical we distinguish the two
isomers by adding the prefix cis (same side) or trans
(opposite sides)
Example
H
H
Cl
Cl
cis-1,2-dicholroethene
b.p = 60 oC
m.p. = -80 oC
H
Cl
Cl
H
H
H3C
trans-1,2-dicholroethene
47 oC
-50 oC
(depends on polarity)
(depends on symmetry)
H
CH3
cis-2-butene
H
H3C
trans-2-butene
CH3
H
If the groups attached to the double bond are different, we
distinguish the two isomers by adding the prefix Z (same side)
or E (opposite sides) depending on the atomic number of the
atoms attached to each end of the double bond
I
Cl
Br
CH3
I
CH3
Br
Cl
Z-2-bromo-1-chloro-1-iodopropene
E-2-bromo-1-chloro-1-iodopropene
O
CH3
HC
CH3
I
CH2
Cl
H
HO
Z
Z
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
CH3
2-methyl-2-propanol
tert-butyl alcohol
2-propen-1-ol
allyl alcohol
Assigning Priority
Halogens < alkanes < alkenes (alkynes) < amines
< OH < ketone < aldehyde < acid < ester
NH2
CH2 = CH - CH2OH
H3C
2-propen-1-ol
allyl alcohol
C
C
C
H2
CH2OH
OH
3-pentyn-1-ol
4-amino-1-pentanol
OH
OH
HO
cyclopentanol
OH
cis-1,2-cyclopentandiol
2-phenylethanol
Nomenclature of Aldehydes and Ketones
Common aldehydes
O
H
O
H
Methanl
(formaldehyde)
H3C
O
O
H
CH3CH2
ethanl
(acetaldehyde)
H
propanal
(propionaldehyde)
CH3CH2CH2
butanal
(n-butyraldehyde)
O
O
H
O
H
H
H
OH
OMe
benzaldehyde
salicylaldehyde
(2-hydroxybenzenecarbaldehyde)
O
H
cyclopentanecarbaldehyde
OH
Vanillin
Common Ketones
O
O
CH3
H3C
propanone
(acetone)
O
cyclohexanone
H3C
O
CH3
CH3
H3C
2-butanone
3-pentanone
(ethyl methyl ketone) (diethyl ketone)
H3C
O
acetophenone
(methyl phenyl ketone)
O
benzophenone
(diphenyl ketone)
Nomenclature of aldehydes and ketones
(al) aldehyde,
(one) ketone
alkanes < alkenes < OH < ketone < aldehyde < acid < ester
Examples
CH3 CH3
3
4
1 2
5
O
CH3 H
2
O
H3C 3
1
4
Cl
2,4-dimethyl-3-hexanone
2-cholro-3-methylbutanal
OH
5 4
3
O
O
2
H 1
1
4-hydroxy-2-pentanone
6
O
3 CH3
2
4
3-oxobutanl
6
5
3 2
4
1
CH3
O
3-hexen-2-one
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.
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.
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.
Mechanism of Substitution
SN2 vs SN1 Reactions
SN2 1 > 2 > 3 (due to Steric factor)
Exception !!!!!!!
SN1
3 > 2 > 1 (due to dispersal of charge)
Very slow reaction
Rearrangement
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
Formation of Alkyl sulfonates
Sulfonates are good leaving groups
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
Glycerol (glycerine)
(1,2,3-propantriol)
b. p. 290 oC
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
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)
Testing ethers for the presence of peroxides
CH3CH2OCH2CH3
+
O2
CH3CH2OCHCH3
OOH
Peroxide
FeSO4
Fe3+
SCNRed colour
If the ether contains solid wastes, this means it contains a lot of peroxides,
and the ether must be discarded
Ethers containing small amounts of peroxides can be treated with
(FeSO4) to get red of the peroxide
The Grignard Reagent
R
R-X
+ Mg
..
O
R
H2O, ROH
..
dry ether
R
R
H
H+, RNH
Mg X
..
R
O
..
R
D2O
R
Grignard reagent
alkyl magnesium halide
It acts as a Lewis base
Me - Mg - Br
Ph - Mg - Cl
=
=
methyl magnesium bromide
phenyl magnesium chloride
Example
Br
D
1) Mg / dry ether
HBr
2) D2O
PBr3
or HBr
Br
D
Preparation of Ethers
A)
Symmetrical ethers (from alcohols)
2 Et - OH
H2SO4
Et - O - Et
140 oC
B)
Unsymmetrical ethers (Williamson’s Synthesis)
R - OH
Na
R O- Na+
+
R - O - R'
R' - X
primary
Example
Na
Ph - OH
Ph O- Na+
+
CH3CH2 - X
Ph - O - CH2CH3
primary
How can you prepare the following ether? (Choose the correct RX)
CH3
H3C
O
CH3
CH3
Cleavage of Ethers
Ethers are cleaved by strong acids such as HI
Ph - O - CH2CH3
HI
Ph - OH
+
I- CH2CH3
Iodine attacks the primary carbon
CH3
H3C
O
CH3
HI
CH3
CH3
H3C
OH
CH3
+
ICH2
CH3
Epoxides (Oxiranes) Cyclic ethers with a three-membered rings
O
O
O
H3C
C
H2
H3C
ethylene oxide
(oxirane)
H
propylene oxide
(methyloxirane)
O
H3C
1-butene oxide
(ethyloxirane)
H
H
CH3
H3C
cis-2-butene oxide
(cis-2,3-dimethyloxirane)
O
CH3
H
trans-2-butene oxide
(trans-2,3-dimethyloxirane)
Preparation
Ag catalyst
CH2 = CH2
+
O
O2
250 oC, Pressure
O
+
R-C-O-O-H
peroxy acid
O
Reactions of Epoxides
When attacked by nucleophiles, epoxides undergo acid catalized ring opening.
H2O
H
CH2CH2OH
OMe
MeOH
CH2CH2OH
H+
O
HOCH2CH2OH
OCH2CH2OH
CH2CH2OH
1) PhMgBr
2) H2O
Cl
1) PhMgBr
CH2CH2OH
2) H2O
Ph
CH2CH2OH
Ph
CH2CH2OH
Cyclic Ethers
O
O
O
tetrahydrofurane
(THF)
tetrahydropyran
O
dioxane
O
O
O
O
O
O
O
O
O
12-Crown-4
O
O
O
M
O
O
O
O
O
O
O
15-Crown-5
O
O
18-Crown-6
cavity diameter = 2.6 - 3.2 A
Ion diameter
Na+ = 1.90 A
K+ = 2.66 A
Cs+ = 3.34 A
only K+ fits in the cavity
18-Crown-6