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Classification, Physical and Chemical Properties of Alkanols Chemistry Tutorial

Key Concepts

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Classification of Alkanols

Alkanols can be classified as primary, secondary or tertiary depending on the location of the OH (hydroxyl or hydroxy) functional group.
Chemists use o notation to refer to primary, secondary and tertiary alkanols:

The general structure of primary, secondary and tertiary alkanols is summarised in the table below:
(Note that R, R', R" represent alkyl, CnH2n+1, chains)

Classification (o) General Formula Location of -OH group
Primary 1o
  H
|
 
R-C-OH
  |
H
 
-OH on a terminal (end) carbon atom

Secondary 2o
  H
|
 
R-C-OH
  |
R'
 
-OH on a carbon atom is bonded to 2 other carbon atoms

Tertiary 3o
  R"
|
 
R-C-OH
  |
R'
 
-OH on a carbon atom is bonded to 3 other carbon atoms

The table below gives examples of primary, secondary and tertiary alkanols:

Classification (o) General Formula Examples
Primary 1o
  H
|
 
R-C-OH
  |
H
 
  H
|
 
CH3-CH2-CH2-C-OH
  |
H
 

butan-1-ol
(or 1-butanol)
  H
|
 
CH3-CH2-CH2-CH2-C-OH
  |
H
 

pentan-1-ol
(or 1-pentanol)

Secondary 2o
  H
|
 
R-C-OH
  |
R'
 
  H
|
 
CH3-CH2-C-OH
  |
CH3
 

butan-2-ol
(or 2-butanol)
  HO
|
 
CH3-CH2-C-CH2-CH3
  |
H
 

pentan-3-ol
(or 3-pentanol)

Tertiary 3o
  R"
|
 
R-C-OH
  |
R'
 
  CH3
|
 
CH3-C-OH
  |
CH3
 

2-methylpropan-2-ol
(or 2-methyl-2-propanol)
  OH
|
 
CH3-CH2-C-CH2-CH3
  |
CH3
 

3-methylpentan-3-ol
(or 3-methyl-3-pentanol)

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Physical Properties of Alkanols

Alkanols are polar molecules. This effects their physcial properties.

Boiling Point

Alkanols are polar molecules: R- Oδ--Hδ+
where the carbon chain is represented by R and δ- represents a partial negative charge on the oxygen atom and δ+ represents a partial positive charge on the hydrogen atom attached to the oxygen atom of the alkanol.
Hydrogen bonding can therefore occur between the alkanol molecules as shown below:

R-Oδ--Hδ+where - represents a covalent bond between atoms within the molecule
and .. represents the hydrogen bond between molecules

In order to boil an alkanol, enough energy must be supplied to break both

  1. the stronger hydrogen bonds between the polar parts of the molecules and
  2. the weaker intermolecular forces (london or dispersion forces) between the non-polar carbon (alkyl) chains (R).
.
.
.
.
.
.
Hδ+-Oδ--R

The boiling points of some primary alkanols are given in the table below. Can you see a trend in the boiling points of these primary alkanols?

Primary Alkanols
Preferred IUPAC name
(alternative IUPAC name)
Functional name Semi-Structural Formula Boiling
Point
(°C)
Trend
methanol methyl alcohol CH3-OH 65 lowest
ethanol ethyl alcohol CH3-CH2-OH 78
propan-1-ol
(1-propanol)
n-propyl alcohol CH3-CH2-CH2-OH 97
butan-1-ol
(1-butanol)
n-butyl alcohol CH3-CH2-CH2-CH2-OH 117
pentan-1-ol
(1-pentanol)
n-pentyl alcohol CH3-CH2-CH2-CH2-CH2-OH 138
hexan-1-ol
(1-hexanol)
n-hexyl alcohol CH3-CH2-CH2-CH2-CH2-CH2-OH 157 highest

As the carbon chain gets longer (molecular mass increases) the boiling point increases.
As the non-polar carbon chain length increases, the weak intermolecular forces (dispersion or london forces) holding the chains together weakly becomes increasingly significant so more energy is required to separate the molecules.

Compare the boiling point of each alkanol with the boiling point of its parent alkane as shown in the table below. Can you see a pattern, or trend, in the data?

number of
carbon atoms
in carbon chain
boiling point (°C)
alkane alkan-1-ol
1       meth -162 65
2       eth -88.6 78
3       prop -42.1 97
4       but -0.5 117
5       pent 36.1 138
6       hex 68.7 157

You can see the trend clearly if you graph the data as shown below:

Temperature
(°C)
Boiling Point of Alkanes and Primary Alkanols

Number of carbon atoms

The boiling point of an alkanol is higher than the boiling point of the corresponding alkane because the energy required to break the hydrogen bonds between alkanol molecules is greater than the energy required to break the weak intermolecular forces between alkane molecules.

Increasing the number of polar OH (hydroxyl or hydroxy) functional groups on alkanol molecules increases the boiling point of the alkanol as shown in the table below:

name formula boiling
point (°C)
  name formula boiling
point (°C)
ethanol
(ethyl alcohol)
CH3-CH2-OH 78   propan-1-ol
(1-propanol)
CH3-CH2-CH2-OH 97
ethane-1,2-diol
(1,2-ethanediol)
(ethylene glycol)
HO-CH2-CH2-OH 197   propane-1,2,3-triol
(1,2,3-propanetriol)
(glycerol)
  H
|
  H
|
  H
|
 
HO-C-C-C-OH
  |
H
  |
OH
  |
H
 
290

Ethane-1,2-diol (1,2-ethanediol or ethylene glycol) has more OH (hydroxyl or hydroxy) functional groups than ethanol.
More OH functional groups means that more hydrogen bonds can form between the molecules.
Since hydrogen bonds are a stronger intermolecular force than the dispersion (london) forces that act between the non-polar alkyl chains, more energy will be required to separate molecules of ethane-1,2-diol than needed to separate molecules of ethanol.
Ethane-1,2-diol has a higher boiling point that ethanol.

Similarly, propane-1,2,3-triol (1,2,3-propanetriol or glycerol) has 3 OH functional groups while propan-1-ol (1-propanol) has only 1 OH functional group.
More OH functional groups means that more hydrogen bonds can form between molecules of propane-1,2,3-triol than can form between molecules of propan-1-ol.
More energy will be required to separate molecules of propane-1,2,3-triol.
Propane-1,2,3-triol has a higher boiling point than propan-1-ol.

Solubility

The table below gives the solubility of some primary alkanols in water. Can you see a pattern, or trend, in the data?

Preferred IUPAC name
(alternative IUPAC name)
formula Solubility
(g/100g water)
Trend
methanol CH3-OH miscible  
ethanol CH3-CH2-OH miscible  
propan-1-ol
(1-propanol)
CH3-CH2-CH2-OH miscible  
butan-1-ol
(1-butanol)
CH3-CH2-CH2-CH2-OH 8 more soluble
pentan-1-ol
(1-pentanol)
CH3-CH2-CH2-CH2-CH2-OH 2.3
hexan-1-ol
(1-hexanol)
CH3-CH2-CH2-CH2-CH2-CH2-OH 0.6 less soluble

For short chain alkanols, the ability of these alkanols to form hydrogen bonds with the polar water molecules is responsible for them being soluble in water (miscible with water), as shown in the diagram below:

    R-Oδ--Hδ+
       .
.
.
   
H-Oδ--Hδ+    

As the number of carbon atoms in the carbon (alkyl) chain increases however, the weak intermolecular forces (London or Dispersion forces) acting between the non-polar alkyl chains become increasingly important.
Long non-polar alkyl chains are more attracted to each other than they are to the polar water molecules, so that the solubility of the alkanol molecules in water decreases.

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Chemical Properties of Alkanols

The OH functional group (hydroxyl functional group) is the active site on alkanol molecules.

Alkanols can undergo a number of chemical reactions including:

⚛ complete combustion in excess oxygen to produce carbon dioxide and water.
Energy is also produced during the combustion of alkanols, making alkanols useful as fuels.

⚛ reaction with active metals to produce a salt (called a metal alkanolate2) and hydrogen gas.

⚛ reaction with alkanoic acids to produce esters.

⚛ elimination reactions to produce alkenes (dehydration of alkanols) in the presence of a strong acid such as sulfuric acid

oxidation reactions (primary and secondary alkanols ONLY).

Combustion of Alkanols

Energy is released during the combustion of alkanols according to the following general chemical equation:

alkanol + excess oxygen gas → carbon dioxide + water + energy

The table below gives the amount of energy in kJ mol-1 that is released when 1 mole of each of the alkanols is combusted in excess oxygen.
Can you see a pattern, or trend, in the data?

no.carbon
atoms in
the chain
primary
alkanol
molecular
mass
energy released
(kJ mol-1)
1 methanol
CH3OH
32 726
2 ethanol
CH3CH2OH
46 1367
3 propan-1-ol
CH3CH2CH2OH
60 2021
4 butan-1-ol
CH3CH2CH2CH2OH
74 2671
5 pentan-1-ol
CH3CH2CH2CH2CH2OH
88 3331

Increasing the length of the carbon chain attached to the OH (hydroxyl or hydroxy) functional group increases the amount of energy released when the primary alkanol combusts.

CnH2n+1OH+3n/2O2(g) nCO2(g)+(n+1)H2O+ energy
CH3-OH+3/2O2(g) CO2(g)+2H2O+ 726 kJ/mol
C2H5-OH+3O2(g) 2CO2(g)+3H2O+ 1367 kJ/mol
C3H7-OH+9/2O2(g) 3CO2(g)+4H2O+ 2021 kJ/mol
C4H9-OH+6O2(g) 4CO2(g)+5H2O+ 2671 kJ/mol

If we graph the data the trend in energy released by combustion of these alkanols becomes even more obvious:

Energy released
during combustion
(kJ mol-1)
Heat of Combustion of Primary Alkanols
(methanol to pentan-1-ol)


Number of Carbon Atoms in Chain

Slope (gradient) of the graph ≈ 650
(kJ mol-1 per carbon atom)
Energy released (kJ mol-1) ≈ no. C atoms x 650
Example, CH3OH, 1 C atom,
    energy released ≈ 1 x 650 ≈ 650 kJ mol-1
Example, C2H5OH, 2 C atoms,
    energy released ≈ 2 x 650 ;≈ 1300 kJ mol-1
Example, C3H7OH, 3 C atoms,
    energy released ≈ 3 x 650  ≈ 1950 kJ mol-1

Increasing the length of the carbon chain by 1 carbon atom increases the amount of energy released during complete combustion by about 650 kJ mol-1.

Incomplete combustion of an alkanol produces water and either carbon monoxide or solid carbon or both carbon monoxide and solid carbon.

Reaction with Active Metal

Alkanols react with active metals to produce a salt and hydrogen gas as shown by the general chemical equation below:

alkanol + active metal → salt + hydrogen gas

Active metals include Group 1 (IA or alkali) metals such as sodium and potassium.

The salt of an alkanol in IUPAC nomenclature is called a metal alkanolate (traditional name is a metal alkoxide).
The alkanolate ion has the general formula R-O-.
These metal alkanolate salts are named the same way as inorganic salts:

Some examples of alkanols and their corresponding alkanolate ions are given in the table below:

alkanol alkanolate ion
(alkoxide ion)
methanol
CH3-OH
methanolate
(methoxide)
CH3-O-

ethanol
CH3-CH2-OH
ethanolate
(ethoxide)
CH3-CH2-O-

propan-1-ol
(1-propanol)
CH3-CH2-CH2-OH
propan-1-olate
(propoxide)
CH3-CH2-CH2-O-

butan-1-ol
(1-butanol)
CH3-CH2-CH2-CH2-OH
butan-1-olate
(butoxide)
CH3-CH2-CH2-CH2-O-

For example, ethanol, CH3CH2OH reacts with sodium metal to produce sodium ethanolate, and reacts with potassium metal to produce potassium ethanolate, as shown in the balanced chemical equations below:

general word equation: alkanol + active metal alkanolate + hydrogen gas
general chemical equation: R-OH + M R-O-M+ + ½H2(g)

word equation example: ethanol + sodium sodium ethanolate + hydrogen gas
chemical equation example: CH3CH2-OH + Na(s) CH3CH2-O-Na+ + ½H2(g)

word equation example: ethanol + potassium potassium ethanolate + hydrogen gas
chemical equation example: CH3CH2-OH + K(s) CH3CH2-O-K+ + ½H2(g)

The longer the carbon chain of the alkanol, the less vigorous the reaction between the alkanol and the active metal.
For example, sodium reacts readily with ethanol, but only sluggishly with butan-1-ol (1-butanol).

Alkanols React with Alkanoic Acids

Alkanols react with alkanoic acids to produce esters.
For this reason, the reaction between alkanols and alkanoic acids are usually referred to as esterification reactions.

The table below gives examples of the reaction between primary alkanols and alkanoic acids to produce esters:

general word equation: primary alkanol + alkanoic acid ester + water
general chemical equation: R-OH + R'-COOH R'COOR + H2O

word equation example: ethanol+propanoic acidethyl propanoate+water
chemical equation example: C2H5-OH+C2H5-COOHC2H5-COOC2H5+H2O

word equation example: propan-1-ol
(1-propanol)
+acetic acid(3)
(ethanoic acid)
propyl acetate(4)
(propyl ethanoate)
+water
chemical equation example: C3H7-OH+CH3-COOHCH3-COOC3H7+H2O

Dehydration of Alkanols

The dehydration of alkanols is an example of an elimination reaction.
Water is eliminated during the reaction.

Examples of the dehydration of primary alkanols using hot, concentrated sulfuric acid are given below:

general word equation: alkanolhot conc sulfuric acid
--------------------->
alkene+water

word equation example: ethanolhot conc sulfuric acid
--------------------->
ethene+water
chemical equation example: CH3CH2OHhot conc H2SO4
--------------------->
CH2=CH2+H2O

word equation example: propan-1-ol
(1-propanol)
hot conc sulfuric acid
--------------------->
prop-1-ene
(propene)
+water
chemical equation example: CH3CH2CH2OHhot conc H2SO4
--------------------->
CH3CH=CH2+H2O

Oxidation of Alkanols

Primary alkanols can be oxidised using a strong oxidising agent such as potassium permanganate solution or potassium dichromate solution..
Primary alkanols are first oxidised to the alkanal (aldehyde) which undergoes further oxidation to produce the alkanoic acid as shown below:

general word equation: 1o alkanoloxidising agent
------------------>
alkanal
(aldehyde)
oxidising agent
------------------->
alkanoic acid
(carboxylic acid)

word equation example: ethanoloxidising agent
------------------>
acetaldehyde(5)
(ethanal)
oxidising agent
------------------>
acetic acid
(ethanoic acid)
chemical equation example: CH3CH2OH[O]
------------------>
CH2-HC=O[O]
------------------>
CH3-COOH

word equation example: propan-1-ol
(1-propanol)
oxidising agent
------------------>
propanaloxidising agent
------------------>
propanoic acid
chemical equation example: CH3CH2CH2OH[O]
------------------>
CH3CH2-HC=O[O]
------------------>
CH3CH2-COOH

Secondary alkanols can be oxidised using a strong oxidising agent such as potassium permanganate solution or potassium dichromate solution.
The oxidation of a secondary alkanol produces an alkanone (ketone) as shown in the chemical equations below:

general word equation: 2o alkanoloxidising agent
------------------>
alkanone
(ketone)

word equation example: propan-2-ol
(2-propanol)
oxidising agent
------------------>
acetone(6)
(propan-2-one)
chemical equation example:
H
|
CH3-C-OH
|
CH3
[O]
------------------>
CH3-C=O
|
CH3

word equation example: butan-2-ol
(2-butanol)
oxidising agent
------------------>
butan-2-one
(butanone)
chemical equation example:
H
|
CH3-CH2-C-OH
|
CH3
[O]
------------------>
CH3-CH2-C=O
|
CH3

Tertiary alkanols cannot be oxidised using oxidising agents such as permanganate or dichromate solutions.

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Footnotes:

(1) The OH functional group in alkanols is called the hydroxy group or hydroxyl group.
While the IUPAC document refers to the hydroxy group, hydroxyl is also a possible name because of the retention of the unpaired electron on the oxygen atom.
Hydroxide is NOT a possible name for the OH group because the "ide" suffix refers to the gain of an electron, that is, it refers to the negatively charged OH- ion.
Note that when another functional group takes precedence over the OH functional group, the OH group is then named as the hydroxy group.

(2) Preferred IUPAC names will be given throughout this tutorial, acceptable IUPAC alternative nomenclature will be given in (parentheses).
The rules for naming organic compounds are still being developed. The most recent document for referral is "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004).

(3) The traditional name of acetic acid is the preferred IUPAC name for CH3COOH instead of the more systematic IUPAC name of ethanoic acid.

(4) Because the preferred IUPAC name for CH3COOH is acetic acid and NOT ethanoic acid, the preferrred IUPAC name for the ion is acetate NOT ethanoate.

(5) Because the preferred IUPAC name for CH3COOH is acetic acid and NOT ethanoic acid, the preferrred IUPAC name for the aldehyde is acetaldehyde NOT ethanal.

(6) The preferred IUPAC name for (CH3)2C=O is the traditional name of acetone and NOT the more systematic names of propan-2-one nor propanone.