Structures of Alcohols, Phenols, Thiols and Ethers
• Alcohols, phenols, thiols and ethers consist of a
hydrocarbon singly bonded to an oxygen or a sulfur
• Alcohols have an -OH group attached to an alkane,
phenols have an -OH group attached to a benzene, thiols
have an -SH group attached to an alkane and ethers have
an O bonded to two C’s
OH
OH
Alchol
SH
Thiol
Phenol
O
Ether
Naming Alcohols
• Parent name ends in -ol
• Find longest chain containing the C to which the OH
group is attached
• Number C’s starting at end nearest OH group
• Locate and number substituents and give full name
- use a number to indicate position of OH group
- cyclic alcohols have cyclo- before the parent name;
numbering begins at the OH group, going in
direction that gives substituents lowest possible
numbers
- use a prefix (di-, tri-) to indicate multiple OH
groups in a compound
Classification of Alcohols
• Alcohols can be classified as methyl, primary,
secondary or tertiary
• Classification is based on the number of alkyl
groups attached to the carbon to which the OH
group is attached
• If OH is attached to a 1 C, it’s a 1 alcohol, etc.
H
H
C
H
OH
H
Methyl
H 3C
C
CH3
CH3
OH
H 3C
C
OH
H
H
Primary
Secondary
H 3C
C
CH3
Tertiary
OH
Naming Phenols
• Phenol is the common name for an OH group
attached to a benzene, and is accepted by IUPAC
• Compounds with additional substituents are
named as substituted phenols
• Ortho, meta and para are used when there is only
one other substituent
• If there are two or more additional substituents,
each must be numbered, beginning at the OH and
going in direction that gives substituents lowest
numbers (or alphabetical if same in both
directions)
Many phenols have pleasant odors, and some are bioactive
- Euganol (from cloves) is a topical anesthetic
- Thymol (from thyme) is an antiseptic
Naming Thiols
• Parent name ends in -thiol
• Find longest chain containing the C to which the SH
group is attached
• Number C’s starting at end nearest SH group
• Parent name is alkane name of carbon portion of
longest chain, followed by thiol
• Locate and number substituents and give full name
- use a number to indicate position of SH group
- cyclic thiols have cyclo- before the parent name;
numbering begins at the SH group, going in direction
that gives substituents lowest possible numbers
- use a prefix (di-, tri-) to indicate multiple SH groups
in a compound
Naming Ethers
• Simple ethers are named by their common names
• For common names: name each alkyl group
attached to the oxygen followed by ether
• For complex ethers IUPAC names are used
• For IUPAC names:
1. Name as an alkane, with larger alkyl group
being the parent chain
2. The smaller alkyl group and the O are named
together as an alkoxy group (replace -yl with -oxy)
3. Number chain starting at end nearest alkoxy
group
4. Use a number to give location of alkoxy group
Naming Cyclic Ethers
• Cyclic ethers are generally named by their common names
(we will not study the IUPAC names)
• A cyclic ether containing two carbons is called ethylene
oxide (generally known as epoxides)
• A cyclic ether containing 4 carbons (with 2 double bonds)
is called a furan
• A cyclic ether containing 5 carbons (with 2 double bonds)
is called a pyran
• A cyclic ether containing 4 carbons and 2 oxygens is called
a dioxane
O
O
O
O
ethylene oxide
furan
tetrahydrofuran
O
O
O
pyran
tetrahydropyran
1,4-dioxane
Naming Examples
OH
OH
OH
SH
2-propanol
2-ethyl-4-methylcyclopentanol
propanethiol
OH
CH3
ortho-methylphenol
O
O
Br
Br
2,4-dibromophenol
O
diethyl ether
O
furan
1,4-dioxane
Physical Properties of Alcohols, Phenols, Thiols and Ethers
• All of these types of compounds have a bent geometry around the
O or the S, and are polar compounds
• Alcohols and phenols contain a very polarized O-H bond, and they
can H-bond with themselves and with other alcohols or water
- Small alcohols (4 or less C’s) are soluble in water
- Phenol is soluble in water (even with 6 C’s) because it partially
ionizes in water (it’s a weak acid)
- Alcohols and phenols have relatively high boiling points
• Thiols are much less polar than alcohols because the
electronegativity of S is the same as that of C (2.5), much less than
that of O (3.5), so C-S and S-H bonds are not polar
- thiols do not H-bond and have relatively low boiling points
• Ethers do not H-bond with themselves, so have boiling points
similar to hydrocarbons
-ethers are only slightly soluble in water and are highly flammable
Acidity and Basicity of Alcohols and Phenols
• Alcohols and phenols, like water, can act as either weak acids or
weak bases (although phenol is more acidic)
• Phenols are more acidic because the anion that forms upon loss
of the proton is stabilized by resonance
CH4
1.0
H2O
1.8 x 10-16
H
CH3CH2SH
1.0 x 10-12
Phenol
1.2 x 10-10
CH3CO2H
1.8 x 10-5
1.0
O
HCl
+
Cl
+
NH 4
+
H 3O
H
H
x 10-16
1.0
+
O
x 10-50
CH3CH2OH
HCl
H
Ka
Compound
+
O
NH 3
O
O
O
H
+
H 2O
x 107
O
O
O
O
Combustion Reactions of Alcohols and Ethers
• Both alcohols and ethers can burn with oxygen to
produce water, carbon dioxide and heat (just like
hydrocarbons)
• However, ethers are much more flammable than
alcohols and care should be taken when working with
ethers in the laboratory (just a spark from static
electricity can set off ether fumes)
Examples:
CH3CH2OH + 3O2  2CO2 + 3H2O + Heat
CH3-O-CH3 + 3O2  2CO2 + 3H2O + Heat
Dehydration of Alcohols to Form Alkenes
• An alcohol can lose a water molecule to form an alkene using an acid
catalyst such as H2SO4 and heat (an “elimination reaction”)
• This is the reverse of the addition of H2O to an alkene
• Dehydration is favored by using heat (endothermic reaction) and a
solvent other than water (lower concentration of H2O)
• When more than one alkene can be formed, Zaitsev’s rule states that
the more substituted alkene will be the major product
• Order of reactivity = 3 > 2 > (1 > methyl)
- In fact this reaction only works with 3 and 2 alcohols
OH
H 3O+
+
+
H2O
Heat
H
OH
H 3O+
H
CH3
+
CH3
Heat
+
H2O
Mechanism of Acid-Catalyzed Dehydration of an Alcohol
• First, the acid catalyst protonates the alcohol
• Next, H2O is eliminated to form a carbocation
• Finally, a proton is removed to form an alkene + H3O+
H
O
+
O
+
O
H
H
H
H
H
H
H
OH
H
O
+
O
H
H
H
+
H
+
O
H
H
O
H
H