Properties of Alkanenitriles Chemistry Tutorial

Key Concepts

• Alkanenitriles contain the polar C≡N functional group (cyano functional group)

  ^δ+C≡N^δ-

• Physical properties of Alkanentriles (R-C≡N)

  ⚛ Short chain alkanenitriles are liquid at room temperature

  Boiling points increase as the length of the carbon chain increases.

  ⚛ Short chain alkanenitriles are soluble in water

  Solubility decrease as the length of the carbon chain increases.

• Chemical properties of Alkanentriles (R-C≡N)

  ⚛ Reduction to produce alkanamines

  R-C≡N + 2H₂ → R-CH₂-NH₂

  ⚛ Acid hydrolysis to produce alkanoic acid

  R-C≡N + 2H₂O + H⁺ → R-COOH + NH₄⁺

  ⚛ Base hydrolysis to produce the salt of an alkanoic acid (alkanoate)

  R-C≡N + H₂O + OH^- → R-COO^- + NH₃

• Synthesis of Alkanentriles (R-C≡N)

  ⚛ Substitution of halogen in a 1-haloalkane by cyanide

  R-CH₂X + CN^- → R-CH₂-C≡N + X^-

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Physical Properties of Alkanenitriles

The cyano functional group is polar because nitrogen is more electronegative than carbon.
The negatively charged electrons in the covalent bond between carbon and nitrogen are more strongly attracted to the nitrogen atom than they are to the carbon atom.
The nitrogen atom therefore carries a partial negative charge while the carbon atom carries a partial positive charge, as shown below:

^δ+C≡N^δ-

The polar nature of the C≡N functional group, and the length of non-polar hydrocarbon chain attached to it, determine the physical properties of alkanenitriles such as boiling point and solubility.

Boiling Point of Alkanenitriles

Consider the boiling points of these short-chain alkanenitriles which exist as liquids at room temperature and pressure:

Note that the boiling point of the alkanenitriles increases as the length of the hydrocarbon chain increases.
In the pure substance, one alkanenitrile molecule is attracted to another alkanenitrile molecule by two intermolecular forces of attraction:

• dipole-dipole interactions
• weak intermolecular forces (dispersion forces or London forces)

The stronger dipole-dipole interaction acts between the polar ^δ+C≡N^δ- functional groups.
In the diagram below the dipole-dipole interaction is shown by a dotted line (⚫⚫⚫⚫):

When the hydrocarbon chain is short, this is the most important attractive force acting between the molecules, but as the length of the non-polar hydrocarbon chain increases then the influence of the weaker dispersion forces (or London forces) acting between these long "tails" becomes increasingly significant, resulting in higher boiling points.
That is, as the length of the hydrocarbon chain increases, it requires more energy to weaken these forces of attraction so that the molecules can escape from the liquid and enter the gas phase.

Let's compare the boiling points of these polar alkanetriles to the corresponding non-polar alkanes, and polar primary alkanols, as shown in the graph below:

Notice that the boiling points of polar alkanenitriles and the corresponding polar primary alkanol are very similar.
Even though the generally "stronger" hydrogen-bonds act between alkanol molecules, while the relatively "weaker" dipole-dipole interactions act between alkanenitrile molecules, we have to remember that there are 3 polar covalent bonds in the C≡N functional group which means this functional group is going to be very polar indeed!

Note that the boiling points of polar alkanenitriles are greater than the corresponding non-polar alkane because the intermolecular forces acting between polar molecules (dipole-dipole interactions) are stronger than the weak dispersion forces (London forces) that act between non-polar alkane molecules.

Solubility of Alkanenitriles

Consider the solubility of the following alkanenitriles in water at 20°C

Note that the solubility of alkanenitriles in polar water decreases as the length of the non-polar hydrocarbon chain increases.
While the length of non-polar hydrocarbon chain in an alkanenitrile is short, polar water water molecules can readily form stronger hydrogen-bonds with the the cyano functional groups:

But, as the length of the non-polar hydrocarbon chain increases, the weaker dispersion forces (or London forces) acting between these long "tails" become increasingly significant, making the polar water molecules more attracted to each other than to the longer-chain alkanenitrile. Hence, the solubility of alkanenitriles in water decreases as the length of the hydrocarbon chain increases.

Chemcial Properties of Alkanenitriles

The active site on an alkanenitrile molecule is the cyano functional group (C≡N). The chemical reactions of alkanenitriles are determined by the reactivity of the cyano functional group.

Reduction of Alkanenitriles

In organic chemistry, the "rule of thumb" is that an organic compound has been reduced if it has lost oxygen atoms and/or gained hydrogen atoms.
One way to reduce an organic compound is to react it with hydrogen gas (H₂), which often requires a catalyst as well as heat and pressure. (Note that these reactions are also referred to as hydrogenation reactions because we are adding hydrogen across the CN triple bond).⁽²⁾
Common metallic catalysts include palladium (Pd), platinum (Pt) and nickel (Ni).
When hydrogen is added across the C≡N functional group, the result will be an amine functional group, (-CH₂)-NH₂

For example, acetonitrile (ethanenitrile) can be reduced to ethanamine (ethyl amine) using hydrogen gas and a palladium catalyst as shown below:

For example, butanenitrile can be reduced to butan-1-amine (1-butyl amine) using hydrogen gas and a nickel catalyst as shown below:

Hydrolysis of Alkanenitriles

Hydrolysis refers to a chemical reaction with water.
The reaction between alkanenitriles and water is generally very slow, that is, the hydrolysis of alkanenitriles is slow.
One way to imrpove the rate of the hydrolysis reaction is to perform the reaction under either acidic or basic (alkanine) conditions.

Acid hydrolysis of alkanenitriles

Acid hydrolysis refers to a chemical reaction with water in the presence of an acid.
Hydrochloric acid is used in the acid hydrolysis of alkanenitriles to produce an alkanoic acid.

For example, if acetonitrile (ethanenitrile) is heated under reflux with dilute hydrochloric acid, the products are acetic acid (ethanoic acid) and aqueous ammonium chloride:

Base hydrolysis of alkanenitriles

Base hydrolysis is also known as alkaline hydrolysis and refers to the reaction with water under basic (or alkaline) conditions.
Sodium hydroxide is used in the base hydrolysis of alkanenitriles to produce a sodium salt, sodium alkanoate, and ammonia.

For example, acetonitrile (ethanenitrile) will undergo base hydrolysis using sodium hydroxide to produce sodium acetate (sodium ethanoate) and ammonia:

Synthesis of Alkanenitriles Using 1-Haloalkanes

We will consider the synthesis of alkanenitriles using 1-haloalkanes (halogenoalkane) in a substitution reaction.

The halogen (X) of a 1-haloalkane (R-CH₂X) is a good leaving group, meaning it is relatively easy to induce the halogen atom to leave and be replaced, or substituted, by another functional group such as the cyano functional group (C≡N). ⁽⁴⁾
Inorganic cyanide salts, such as sodium cyanide (NaCN) or potassium cyanide (KCN) are good sources of cyanide ions which will result in the formation of the cyano functional group, C≡N.
However, if we heat a haloalkane under reflux with an aqueous solution of NaCN_(aq) or KCN_(aq) then it is quite likely that a hydrolysis reaction will occur in which an OH functional group will substitute for the halogen atom and we will produce an alkanol!
Therefore it is important to exclude water from the reaction. We can do this by using alcoholic solutions instead of aqueous solutions, or by using dimethyl sulfoxide as the solvent instead of water.⁽⁵⁾ We can dissolve our cyanide salt in ethanol, or, we can dissolve our cyanide salt in diemthyl sulfoxide.

In general, if we heat a haloalkane under reflux with a suitable solution of a cyanide salt, the halogen atom is substituted by the cyanide and the product is an alkanenitrile with a carbon chain that is 1 C atom longer, and, a negatively charged halide ion (X^-):

For example, we could react 1-bromopropane with sodium cyanide in ethanol solution to produce butanenitrile and sodium bromide: