6.2.7: Extending carbon chain length

Question 1

What is cyanide used as when extending the carbon chain length?

Answer 1

-Cyanide is a nucleophile and id attracted to areas of positive charge in a molecule.

Question 2

Give two examples of common nucleophiles other than cyanide.

Answer 2

• H2O (water)

- OH- (hydroxide)

Question 3

The CN- ion is known as the cyanide ion. When does it retain the name cyanide and when is it known as a nitrile functional group.

Answer 3

• It retains the name cyanide in inorganic chemicals such as sodium cyanide (NaCN).
• When found in organic chemicals, it becomes known as a nitrile functional group. e.g. ethanenitrile.

Question 4

Where do haloalkanes have a dipole and what does this result in?

Answer 4

• Haloalkanes have a dipole in the C-X bond due to the significant differences in the electronegativities between carbon and the halogen.
• This means that the carbon is susceptible to nucleophilic attack.

Question 5

How is nucleophilic substitution used to extend the chain length?

Answer 5

-In the laboratory, the haloalkane is mixed with potassium cyanide and heated under reflux.

Question 6

Which solvent used in nucleophilic sub. to extend chain length and why?

Answer 6

• The solvent is ethanol and this ensures that the nucleophile is the cyanide ion.
• If water is used then the hydroxide ion tends to be the nucleophile and the carbon-chain length would not increase.

Question 7

When 1-bromopropane is heated under reflux with potassium cyanide in a solution containing ethanol, the reaction will occur, how does this extend the chain length?

Answer 7

• The mechanism is a nucleophilic substitution where the cyanide takes place of the halogen. .
• The carbon chain has gone from three carbons to four.
• A new C-C bond is formed between the original molecule and the cyanide, which increases the length of a carbon chain by one carbon atom.
• The new nitrile functional group can undergo more reactions and make a variety of different chemicals.

Question 8

When 1-bromopropane is heated under reflux with potassium cyanide in a solution containing ethanol, the reaction will occur, The mechanism is a nucleophilic substitution where the cyanide takes place of the halogen.
What is the balanced symbol equation?
Draw the mechanism.

Answer 8

CH3CH2CH2Br + KCN –> CH3CH2CH2CN + KBr

Question 9

How can hydrogen cyanide and a carbonyl group be used to extend the carbon chain length by nucleophilic addition?

Answer 9

• The carbonyl group has a slight positive charge and this makes it susceptible to nucleophilic attack.
• Hydrogen cyanide can be used to generate the cyanide nucleophile.
• The cyanide ion will be attracted to the carbon of the carbonyl group and will form a new covalent bond which extends the length pf the carbon chain.
• The pi-bond of the C=O then opens and the oxygen accepts the extra pair of electrons.
• This allows the now negatively charged oxygen atom to accept a proton and become a hydroxyl group (-OH).
• This results in a hydroxynitrile orrganic compound,

Question 10

Why is a racemic mixture produced when an asymmetric ketone or any aldehyde other than methanal is used in this reaction (neucleophilic addition using a cyanide ion to extend carbon chain length of a carbonyl group)?

Answer 10

-Because the C=O is a planar bond, so, there is an equally likely chance that the cyanide ion will attack from either side.

Question 11

Why is HCN not usually used in laboratory conditions and what is used instead?

Answer 11

• Because HCN is a highly toxic gas in normal laboratory conditions.
• It stops the mitochondrial enzymes working in living cells.
• A safe alternative in the laboratory is to use a cyanide salt like potassium cyanide (KCN).

Question 12

Why is KCN better to use in labs than HCN?

Answer 12

-It is solid at room temperature and it is easier to handle.

Question 13

Why must the salt (KCN) be used in acidified solution?

Answer 13

-The salt must be used in an acidified solution so that both CN- and H+ ions are present.