Chapter 13- Haloalkanes

Question 1

What is the general formula for a haloalkane with one halogen atom?

Answer 1

CnH2n+1X
Often shortened to R — X

Question 2

How soluble are haloalkanes?

Answer 2

The polar C — X bonds are not polar enough for them to be soluble in water.
The longer the C chain the less soluble in water.
Dissolve in ethanol.

Question 3

How does boiling point vary with different haloalkanes?

Answer 3

Increases with increased chain length.
Increases going down halogen group (larger molecules, VdWs).
Branching reduces melting point.
Higher boiling points than alkanes of similar length due to higher RMM and polarity.

Question 4

How does bond polarity vary with different halogens?

Answer 4

The bond is more polar the higher up the halogen is in the group. Electronegativity of halogen increases going up the group.

Question 5

How does C—X bond enthalpy vary with different halogens?

Answer 5

Decreasing bond strength (bond enthalpy) going down the group as the halogen atom radius increases.
Bond length increases meaning bond is weaker.
Therefore, iodo-compounds are most reactive due to lowest bond enthalpy.

Question 6

How does reactivity of a haloalkane change going down group 7?

Answer 6

Reactivity increases as bond enthalpy decreases which is a more important factor than bond polarity.

Question 7

What is a nucleophile?

Answer 7

A species with a lone pair of electrons to donate

Question 8

What are some example of nucleophiles?

Answer 8

Hydroxide ion -:OH
Ammonia :NH3
Cyanide ion -:CN

Question 9

What is nucleophilic substitution?

Answer 9

A reaction in which a nucleophile attacks a (partial) positive charge to replace an atom or group of atoms

Question 10

What are three examples of nucleophilic substitution reactions?

Answer 10

Haloalkane —> alcohol
Haloalkane —>nitrile
Haloalkane —> amine

Question 11

What does a double headed curly arrow show?

Answer 11

How a pair of electrons move in a reaction

Question 12

What are the conditions needed for a haloalkane to alcohol reaction?

Answer 12

-:OH ions (nucleophile) from aqueous NaOH or KOH (heated to split ions).
Alcohol to act as universal solvent

Question 13

What are the steps of a haloalkane to alcohol reaction?

Answer 13

-:OH ion attacks partial positive C atom bonded to halogen.
Electron pair from covalent bond moves to the halogen, making it a halide ion (which reacts with Na or K from metal hydroxide).
The halide ion is the leaving group.

Question 14

What are the conditions needed for a haloalkane to nitrile reaction?

Answer 14

-:CN ions (nucleophiles) from aqueous NaCN or KCN with alcoholic solution as universal solvent.

Question 15

What are the steps of the haloalkane to nitrile reaction?

Answer 15

Cyanide ion attacks partial positive C atom bonded to halogen.
Electron pair from covalent bond moves to halogen atom, making it a halide ion which is the leaving group (reacting with Na or K).
Produces a nitrile with an extra carbon atom in the chain than haloalkane.

Question 16

What are the conditions needed for a haloalkane to amine reaction?

Answer 16

:NH3 from excess concentrated solution of ammonia in ethanol
Requires pressure

Question 17

What are the steps of the haloalkane to amine reaction?

Answer 17

Ammonia attacks partial positive C atom and electron pair moves to halogen atom, making it a halide ion (leaving group).

Electron pair from one N—H covalent bond moves to the positive N atom and a second NH3 molecule reacts with H+ ion to form an ammonium ion producing an amine and ammonium ion.

Question 18

What is an elimination reaction?

Answer 18

A reaction in which an atom or group of atoms are removed from a molecules, eg a hydrogen halide

Question 19

What are the conditions needed for elimination of a haloalkane?

Answer 19

-:OH ions from NaOH or KOH dissolved in ethanol. These act as a base to remove an H+ ion rather than a nucleophile.
NO WATER present

Question 20

What are the steps in the elimination of a haloalkane?

Answer 20

-:OH ion forms bond with slightly positive H atom on carbon next to C—X bond.
Electron pair from old C—H bond then becomes part of C=C double bond.
Halogen takes pair of electrons from the
C—X bond and leaves as halide ion (leaving group).