1. Structures-
a. Alcohols--->O-H group is attached to a nonaromatic carbon.
(1) May be classified as 1o,2o,3o
depending on the type of Carbon to
which they are attached.
(2) Examples
b. Thiols--> (C-SH)--> mercaptan is an older term for this type of compound.
2. Nomenclature
a. Alcohols-->Common Name=name of
hydrocarbon group + Alcohol
b. Alcohols-->IUPAC-
Select the longest continuous carbon
chain to which the -OH is directly
attached. Drop e from the corresponding alkane name and add the suffix ol.
Number the longest carbon chain so as to give the carbon atom holding the -OH the LOWEST possible number.
Indicate the position of other groups
(substituents)using the lowest series
of numbers relative to the -OH.
If a C=C or triple bond is present its position is given relative to the OH and the position of the OH is set off from the rest of the name by a-followed by the suffix ol.
c. Thiols-->= the patent hydrocarbon name +thiol
Thiol group is more important than alkyl groups attached to the parent chain. Alcohols have a higher naming priority than SH. As a group SH is called mercapto.
B. Physical Properties of Alcohols and Thiols
1. Boiling Point
a. Alcohols have bp’s much HIGHER than ethers or hydrocarbons of similar
molecular weight.
Compound Mole. Wt. bp(oC)
Pentane 72 36
CH3CH2OCH2CH3 74 35
CH3CH2CH2CH2OH 74 118
The increased bp of alcohols is due to
hydrogen bonding between alcohol
molecules.
Energy (Heat) is required to overcome
these attractive forces. Ethers can not
hydrogen bond to each other.
2. Solubility in Water-low molecular weight alcohols are soluble in water.
a. Alcohols can form hydrogen
bonds to water molecules, and thus
they are attracted to water.
b. The solubility DECREASES as the length of the R group INCREASES.
3. Thiols most outstanding characteristic is their bad odor--they stink! The S-H bond is much less polar than the O-H bond, and thus thiols have little tendency to hydrogen bond. For similar molecular mass they have lower boiling points and solubility in water than alcohols.
C. Some Important Alcohols
1. Ethanol-(CH3CH2OH)-most important alcohol
a. Ethanol can be prepared by the fermintation of sugars by yeast. The process
usually stops at 12-15% alcohol=88-85% water.
Yeast
C6H12O6 -------> 2 CH3CH2OH + 2 CO2
Sugar
(1) Azeotrope=constant boiling
MIXTURE-95% ethanol + 5% water
(2) Proof= 2 times alcohol content by volume
(3) Denatured alcohol-unfit to drink
b. Ethanol is prepared from ethene
industrially.
2. Ethylene glycol-HOCH2CH2OH-Used as
antifreeze
D. Reactions of Alcohols
1. General Possibilities-
a. C--O Cleavage
b. O--H Cleavage
2. Alcohols as Acids-Cleavage of the O--H bond
a. Alcohols are weak acids. They are
weaker than water, but much stronger
than any of the hydrocarbons.
Alcohols react with strong bases, Na and K,
to from salts .
(weak acid) (strong base)
b. Formation of Esters
(1) Alkyl Sulfonates-produce good
leaving groups for SN2 reactions.
A sulfonyl chloride A sulfonate ester
(2). The sulfonate group is a better
leaving group than a halogen.
It is often used as the leaving
group in nucleophilic substitution.
(3) Alkyl Phosphates-important in
biochemistry
3. Reactions of the C-O bond
a. Synthesis of Alkenes by Dehydration of Alcohols General Process-Usually El Elimination
H+ = H2SO4, H3PO4, etc
Examples:
Ease of Dehydration Based on Class of
Alcohol-3o > 2o > 1o
Mechanism-E1-Carbocation
(Fast)-Step 1
(SLOW)-Step 2
(Fast)-Step 3
a. The SLOW step in this process is the
formation of the carbocation.
3o Alcohols lead to a 3o carbocation. 3o carbocations are the most stable.
Therefore a 3o alcohol will react
at a faster rate than a 2o or 1o
alcohol.
b. This can be illustrated by the following potential energy diagram.
Molecular Rearrangement-A less stable carbocation will often rearrange its
structure to form a more stable carbocation.
a. Mechanism
Product Distribution
a. The most highly substituted alkene is
formed as the major product(Zaitsev’s
rule).
b. Trans is formed in a greater amount than cis.
c. Rearrangement of the molecular
structure occurs WHENEVER A MORE
STABLE CARBOCATION CAN BE FORMED.
b. Formation of Alkyl Halides
(1) Examples
(2) Mechanisms:
2o, 3o= SN1
1o = SN2
The SN1 Process
The SN2 Process
(3) Primary or Secondary alcohols can
be converted to bromides (PBr3) or
chlorides (SOCl2)
4. Oxidation Reactions
These reactions are useful in synthesis of other compounds and in the identification
of alcohols.
a. Oxidation of PRIMARY Alcohols results in the formation of ALDEHYDES or CARBOXYLIC ACIDS.
b. Oxidation of SECONDARY alcohols produces KETONES.
c. There is reagent which will oxidize a
primary alcohol to the aldehyde and
stop. This reagent does not oxidize
C=C. The reagent is called PCC and
its structure is:
d. Examples:
e. Oxidation is often used to classify
alcohols.
1o and 2o alcohols react rapidly with chromic acid (H2CrO4) giving an orangeto green color change. Tertiary alcohols do not react.
E. Thiols
1. Preparation-usually prepared by SN2 reactions of the highly nucleophilic HS- ion. Only works well when using 1o halides.
Secondary or tertiary halides give mainly b-elimination to form alkenes.
2. Acidity-Thiols are stronger acids than alcohols. Thiols will react with NaOH to form a salt, alchohols will not.
