PJB-2024-110
Structure and classification of alcohols
Fatima Akmal shah
Abstract
Similar to water, an alcohol can be pictured as having an sp3 hybridized tetrahedral oxygen atom with nonbonding pairs of electrons occupying two of the four sp3 hybrid orbitals. (See chemical bonding for a discussion of hybrid orbitals.) Alkyl groups are generally bulkier than hydrogen atoms, however, so the R―O―H bond angle in alcohols is generally larger than the 104.5° H―O―H bond angle in water. For example, the 108.9° bond angle in methanol shows the effect of the methyl group, which is larger than the hydrogen atom of water.
One way of classifying alcohols is based on which carbon atom is bonded to the hydroxyl group. If this carbon is primary (1°, bonded to only one other carbon atom), the compound is a primary alcohol. A secondary alcohol has the hydroxyl group on a secondary (2°) carbon atom, which is bonded to two other carbon atoms. Similarly, a tertiary alcohol has the hydroxyl group on a tertiary (3°) carbon atom, which is bonded to three other carbons. Alcohols are referred to as allylic or benzylic if the hydroxyl group is bonded to an allylic carbon atom (adjacent to a C=C double bond) or a benzylic carbon atom (next to a benzene ring), respectively.
Nomenclature
As with other types of organic compounds, alcohols are named by both formal and common systems. The most generally applicable system is that adopted at a meeting of the International Union of Pure and Applied Chemistry (IUPAC) in Paris in 1957. Using the IUPAC system, the name for an alcohol uses the -ol suffix with the name of the parent alkane, together with a number to give the location of the hydroxyl group. The rules are summarized in a three-step procedure:
Name the longest carbon chain that contains the carbon atom bearing the ―OH group. Drop the final -e from the alkane name, and add the suffix -ol.
Number the longest carbon chain starting at the end nearest the ―OH group, and use the appropriate number, if necessary, to indicate the position of the ―OH group.
Name the substituents, and give their numbers as for an alkane or alkene.
The first example below has a longest chain of six carbon atoms, so the root name is hexanol. The ―OH group is on the third carbon atom, which is indicated by the name 3-hexanol. There is a methyl group on carbon 3 and a chlorine atom on carbon 2. The complete IUPAC name is 2-chloro-3-methyl-3-hexanol. The prefix cyclo- is used for alcohols with cyclic alkyl groups. The hydroxyl group is assumed to be on carbon 1, and the ring is numbered in the direction to give the lowest possible numbers to the other substituents, as in, for example, 2,2-dimethylcyclopentanol.
Common names
The common name of an alcohol combines the name of the alkyl group with the word alcohol. If the alkyl group is complex, the common name becomes awkward and the IUPAC name should be used. Common names often incorporate obsolete terms in the naming of the alkyl group; for example, amyl is frequently used instead of pentyl for a five-carbon chain.
Physical properties of alcohols
Most of the common alcohols are colourless liquids at room temperature. Methyl alcohol, ethyl alcohol, and isopropyl alcohol are free-flowing liquids with fruity odours. The higher alcohols—those containing 4 to 10 carbon atoms—are somewhat viscous, or oily, and they have heavier fruity odours. Some of the highly branched alcohols and many alcohols containing more than 12 carbon atoms are solids at room temperature.
IUPAC name
bp (°C)
density (grams per millilitre)
solubility in water
Physical properties of selected alcohols
IUPAC name
common name
formula
mp (°C)
methanol
65
0.79
miscible
ethanol
78
0.79
miscible
1-propanol
97
0.80
miscible
2-propanol
82
0.79
miscible
1-butanol
118
0.81
9.1%
2-butanol
100
0.81
7.7%
2-methyl-1-propanol
108
0.80
10.0%
2-methyl-2-propanol
83
0.79
miscible
1-pentanol
138
0.82
2.7%
3-methyl-1-butanol
132
0.81
2.0%
2,2-dimethyl-1-propanol
113
0.81
3.5%
cyclopentanol
141
0.95
1-hexanol
156
0.82
0.6%
cyclohexanol
162
0.96
3.6%
1-heptanol
176
0.82
0.1%
1-octanol
194
0.83
1-nonanol
214
0.83
1-decanol
233
0.83
2-propen-1-ol
97
0.86
phenylmethanol
205
1.05
diphenylmethanol
298
triphenylmethanol
380
1.20
*Ph represents the phenyl group, C6H5—.
methanol
methyl alcohol
CH3OH
−97
ethanol
ethyl alcohol
CH3CH2OH
−114
1-propanol
n-propyl alcohol
CH3CH2CH2OH
−126
2-propanol
isopropyl alcohol
(CH3)2CHOH
−89
1-butanol
n-butyl alcohol
CH3(CH2)3OH
−90
2-butanol
sec-butyl alcohol
(CH3)CH(OH)CH2CH3
−114
2-methyl-1-propanol
isobutyl alcohol
(CH3)2CHCH2OH
−108
2-methyl-2-propanol
t-butyl alcohol
(CH3)3COH
25
1-pentanol
n-pentyl alcohol
CH3(CH2)4OH
−79
3-methyl-1-butanol
isopentyl alcohol
(CH3)2CHCH2CH2OH
−117
2,2-dimethyl-1-propanol
neopentyl alcohol
(CH3)3CCH2OH
52
cyclopentanol
cyclopentyl alcohol
cyclo-C5H9OH
−19
1-hexanol
n-hexanol
CH3(CH2)5OH
−52
cyclohexanol
cyclohexyl alcohol
cyclo-C6H11OH
25
1-heptanol
n-heptyl alcohol
CH3(CH2)6OH
−34
1-octanol
n-octyl alcohol
CH3(CH2)7OH
−16
1-nonanol
n-nonyl alcohol
CH3(CH2)8OH
−6
1-decanol
n-decyl alcohol
CH3(CH2)9OH
6
2-propen-1-ol
allyl alcohol
H2C=CH−CH2OH
−129
phenylmethanol
benzyl alcohol
Ph−CH2OH*
−15
diphenylmethanol
diphenylcarbinol
Ph2CHOH*
69
triphenylmethanol
triphenylcarbinol
Ph3COH*
162
The boiling points of alcohols are much higher than those of alkanes with similar molecular weights. For example, ethanol, with a molecular weight (MW) of 46, has a boiling point of 78 °C (173 °F), whereas propane (MW 44) has a boiling point of −42 °C (−44 °F). Such a large difference in boiling points indicates that molecules of ethanol are attracted to one another much more strongly than are propane molecules. Most of this difference results from the ability of ethanol and other alcohols to form intermolecular hydrogen bonds. (See chemical bonding: Intermolecular forces for a discussion of hydrogen bonding.)
Similar to water, an alcohol can be pictured as having an sp3 hybridized tetrahedral oxygen atom with nonbonding pairs of electrons occupying two of the four sp3 hybrid orbitals. (See chemical bonding for a discussion of hybrid orbitals.) Alkyl groups are generally bulkier than hydrogen atoms, however, so the R―O―H bond angle in alcohols is generally larger than the 104.5° H―O―H bond angle in water. For example, the 108.9° bond angle in methanol shows the effect of the methyl group, which is larger than the hydrogen atom of water.
One way of classifying alcohols is based on which carbon atom is bonded to the hydroxyl group. If this carbon is primary (1°, bonded to only one other carbon atom), the compound is a primary alcohol. A secondary alcohol has the hydroxyl group on a secondary (2°) carbon atom, which is bonded to two other carbon atoms. Similarly, a tertiary alcohol has the hydroxyl group on a tertiary (3°) carbon atom, which is bonded to three other carbons. Alcohols are referred to as allylic or benzylic if the hydroxyl group is bonded to an allylic carbon atom (adjacent to a C=C double bond) or a benzylic carbon atom (next to a benzene ring), respectively.
Nomenclature
As with other types of organic compounds, alcohols are named by both formal and common systems. The most generally applicable system is that adopted at a meeting of the International Union of Pure and Applied Chemistry (IUPAC) in Paris in 1957. Using the IUPAC system, the name for an alcohol uses the -ol suffix with the name of the parent alkane, together with a number to give the location of the hydroxyl group. The rules are summarized in a three-step procedure:
Name the longest carbon chain that contains the carbon atom bearing the ―OH group. Drop the final -e from the alkane name, and add the suffix -ol.
Number the longest carbon chain starting at the end nearest the ―OH group, and use the appropriate number, if necessary, to indicate the position of the ―OH group.
Name the substituents, and give their numbers as for an alkane or alkene.
The first example below has a longest chain of six carbon atoms, so the root name is hexanol. The ―OH group is on the third carbon atom, which is indicated by the name 3-hexanol. There is a methyl group on carbon 3 and a chlorine atom on carbon 2. The complete IUPAC name is 2-chloro-3-methyl-3-hexanol. The prefix cyclo- is used for alcohols with cyclic alkyl groups. The hydroxyl group is assumed to be on carbon 1, and the ring is numbered in the direction to give the lowest possible numbers to the other substituents, as in, for example, 2,2-dimethylcyclopentanol.
Common names
The common name of an alcohol combines the name of the alkyl group with the word alcohol. If the alkyl group is complex, the common name becomes awkward and the IUPAC name should be used. Common names often incorporate obsolete terms in the naming of the alkyl group; for example, amyl is frequently used instead of pentyl for a five-carbon chain.
Physical properties of alcohols
Most of the common alcohols are colourless liquids at room temperature. Methyl alcohol, ethyl alcohol, and isopropyl alcohol are free-flowing liquids with fruity odours. The higher alcohols—those containing 4 to 10 carbon atoms—are somewhat viscous, or oily, and they have heavier fruity odours. Some of the highly branched alcohols and many alcohols containing more than 12 carbon atoms are solids at room temperature.
IUPAC name
bp (°C)
density (grams per millilitre)
solubility in water
Physical properties of selected alcohols
IUPAC name
common name
formula
mp (°C)
methanol
65
0.79
miscible
ethanol
78
0.79
miscible
1-propanol
97
0.80
miscible
2-propanol
82
0.79
miscible
1-butanol
118
0.81
9.1%
2-butanol
100
0.81
7.7%
2-methyl-1-propanol
108
0.80
10.0%
2-methyl-2-propanol
83
0.79
miscible
1-pentanol
138
0.82
2.7%
3-methyl-1-butanol
132
0.81
2.0%
2,2-dimethyl-1-propanol
113
0.81
3.5%
cyclopentanol
141
0.95
1-hexanol
156
0.82
0.6%
cyclohexanol
162
0.96
3.6%
1-heptanol
176
0.82
0.1%
1-octanol
194
0.83
1-nonanol
214
0.83
1-decanol
233
0.83
2-propen-1-ol
97
0.86
phenylmethanol
205
1.05
diphenylmethanol
298
triphenylmethanol
380
1.20
*Ph represents the phenyl group, C6H5—.
methanol
methyl alcohol
CH3OH
−97
ethanol
ethyl alcohol
CH3CH2OH
−114
1-propanol
n-propyl alcohol
CH3CH2CH2OH
−126
2-propanol
isopropyl alcohol
(CH3)2CHOH
−89
1-butanol
n-butyl alcohol
CH3(CH2)3OH
−90
2-butanol
sec-butyl alcohol
(CH3)CH(OH)CH2CH3
−114
2-methyl-1-propanol
isobutyl alcohol
(CH3)2CHCH2OH
−108
2-methyl-2-propanol
t-butyl alcohol
(CH3)3COH
25
1-pentanol
n-pentyl alcohol
CH3(CH2)4OH
−79
3-methyl-1-butanol
isopentyl alcohol
(CH3)2CHCH2CH2OH
−117
2,2-dimethyl-1-propanol
neopentyl alcohol
(CH3)3CCH2OH
52
cyclopentanol
cyclopentyl alcohol
cyclo-C5H9OH
−19
1-hexanol
n-hexanol
CH3(CH2)5OH
−52
cyclohexanol
cyclohexyl alcohol
cyclo-C6H11OH
25
1-heptanol
n-heptyl alcohol
CH3(CH2)6OH
−34
1-octanol
n-octyl alcohol
CH3(CH2)7OH
−16
1-nonanol
n-nonyl alcohol
CH3(CH2)8OH
−6
1-decanol
n-decyl alcohol
CH3(CH2)9OH
6
2-propen-1-ol
allyl alcohol
H2C=CH−CH2OH
−129
phenylmethanol
benzyl alcohol
Ph−CH2OH*
−15
diphenylmethanol
diphenylcarbinol
Ph2CHOH*
69
triphenylmethanol
triphenylcarbinol
Ph3COH*
162
The boiling points of alcohols are much higher than those of alkanes with similar molecular weights. For example, ethanol, with a molecular weight (MW) of 46, has a boiling point of 78 °C (173 °F), whereas propane (MW 44) has a boiling point of −42 °C (−44 °F). Such a large difference in boiling points indicates that molecules of ethanol are attracted to one another much more strongly than are propane molecules. Most of this difference results from the ability of ethanol and other alcohols to form intermolecular hydrogen bonds. (See chemical bonding: Intermolecular forces for a discussion of hydrogen bonding.)
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