Classification, Physical and Chemical Properties of Alkanols Chemistry Tutorial

Key Concepts

• Alkanols belong to a group of organic (carbon) compounds known as alcohols.
• Alcohols contain the OH, hydroxyl (or hydroxy)⁽¹⁾, functional group.
• The structure of an alkanol is the same as for the parent alkane EXCEPT that an OH functional group has substituted for one of the hydrogen atoms.
• Depending on the location of the OH (hydroxyl or hydroxy) functional group, alkanols can be classified as one of the following:

  ⚛ primary alcohols (1^o)

  ⚛ secondary alcohols (2^o)

  ⚛ tertiary alcohols (3^o)

• Physical Properties of Alkanols:

  ⚛ Boiling point of primary alkanols increases with increasing length of carbon chain (or molecular mass).
  Boiling point of a primary alkanol is higher than the boiling point of the corresponding alkane.

  ⚛ Simple alkanols are soluble in polar solvents like water.
  Solubility of an alkanol in water decreases with increasing length of carbon chain.

• Chemical Properties of Alkanols:

  ⚛ Alkanols undergo 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.

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

  ⚛ Alkanols react with alkanoic acids to produce esters.

  ⚛ In the presence of a strong acid such as sulfuric acid, alkanols undergo elimination reactions to produce alkenes (dehydration of alkanols).

  ⚛ In the presence of concentrated strong hydrohalic acids (HCl_(aq), HBr_(aq), HI_(aq)), alkanols undergo subsitution reactions to produce a haloalkane (Alkanols and Hydrohalic Acids (HX)).

  ⚛ Primary and secondary alkanols can be oxidised.
  Tertiary alkanols cannot be oxidised.

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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:

• primary alkanol ≡ 1^o alkanol

  -OH on a terminal (end of chain) C atom

• secondary alkanol ≡ 2^o alkanol

  -OH on a C atom bonded to 2 C atoms

• tertiary alkanol ≡ 3^o alkanol

  -OH on a C atom bonded to 3 C atoms

The general structure of primary, secondary and tertiary alkanols is summarised in the table below:
(Note that R, R', R" represent alkyl, C_nH_2n+1, chains)

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

Physical Properties of Alkanols

Alkanols are polar molecules. This effects their physcial properties.

Boiling Point

Alkanols are polar molecules: R- O^δ--H^δ+
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:

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?

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?

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

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:

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?

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:

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.

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

⚛ substitution reactions to produce haloalkanes (alkanols and hydrohalic acid (HX)) in the presence of concentrated strong hydrohalic 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?

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.

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

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:

• the metal is named first
• followed by the name of the alkanolate ion

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

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:

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:

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:

Substitution of Hydroxyl Functional Group by Halide

The hydroxyl functional group (OH) in an alkanol can be replaced by a halogen atom (Cl, Br, or I) in a substitution reaction using concentrated hydrohalic acid (HCl_(aq), HBr_(aq), or HI_(aq)) under suitable conditions:

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:

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:

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