(U2) Halogenoalkanes

Question 1

What is a primary halogenoalkane? (2)

What are the exceptions?

Answer 1

• A halogenoalkane which has 1 carbon atom directly bonded
• to the carbon atom that is bonded to the halogen
• halomethanes

Question 2

What is a secondary halogenoalkane? (2)

Answer 2

• A halogenoalkane which has 2 carbon atoms directly bonded
• to the carbon atom that is bonded to the halogen

Question 3

What is a tertiary halogenoalkane? (2)

Answer 3

• A halogenoalkane which has 3 carbon atoms directly bonded
• to the carbon atom that is bonded to the halogen

Question 4

What is reflux?

Answer 4

Repeated boiling and condensing of a reaction mixture

Question 5

What is hydrolysis? (Chemistry)

Answer 5

Breaking up molecules by reaction with water

Question 6

What is a nucleophile? (3)

Answer 6

• An ion or molecule,
• with a lone pair of electrons,
• that attacks regions of low electron density

Question 7

What is elimination?

Answer 7

A reaction in which a small molecule is removed from a larger molecule

Question 8

What is the general formula of halogenoalkanes?

Answer 8

C_nH_2n+1X

Question 9

What is the trend in the boiling point of halogenoalkanes? (3)

Answer 9

• boiling point increases with no. of halogens - greater VdV
• also increases as the size of the halogen atom increases (e.g. fluoroalkanes would have a lower boiling point than chloroalkanes) - greater VdV
• also the longer the carbon chain, the larger the boiling point

Question 10

[describe the laboratory preparation of a liquid organic compound, such as a halogenoalkane, from the corresponding alcohol]

Answer 10

Preparation of 1-bromobutane

Method:

• 30 cm³ of water, 35 g of powdered sodium bromide and 25 cm³ of butan-1-ol in a 250 cm³ round bottomed flask.
• Fit a tap funnel to the flask via a stillhead.
• 25 cm³ of concentrated sulphuric acid in the tap funnel, and then allow the acid to fall drop wise into the flask, keep contents shaken and cooled in an ice-water bath.
• When the addition is complete, replace tap funnel and stillhead with a reflux water condenser and boil the mixture over a sand-bath for about 45 minutes, shaking the flask often
• Remove the reflux condenser and rearrange apparatus for distillation. Distil off the crude 1-bromobutane (about 30 cm₃).
• Shake the distillate with water in a separating funnel, and run off the lower layer of 1-bromobutane; reject the aqueous layer.
• Return the 1-bromobutane to the funnel, add about half its volume of concentrated hydrochloric acid, and shake. Run off and discard the lower layer of acid.
• Shake the 1-bromobutane cautiously with dilute sodium carbonate solution, cautiously releasing the pressure at intervals.
• Run off the lower layer of 1-bromobutane and add some granular anhydrous calcium chloride. Swirl the mixture until the liquid is clear.
• Filter the 1-bromobutane into a clean dry flask, and distil it, collecting the fraction boiling between 101 - 103ºC.

Question 11

What are the conditions required for the nucleophilic substitution reaction between halogenoalkanes and alkalis?

What are the products?

Answer 11

• alkali must be aqueous - hydroxide ions in aqueous solution
• reacts under reflux with hydroxide ions which come from NaOH or KOH

• an alcohol
• a halide salt

Question 12

What are the conditions required for the reaction between halogenoalkanes and ammonia?

What are the products?

What type of reaction is this?

Answer 12

1. Heat with ethanolic ammonia in a sealed tube
2. An amine (func group = (-NH₂)) and a hydrogen halide, which can react with ammonia to form an ammonium halide
3. Nucleophilic substitution

Question 13

What are the conditions required for the reaction between halogenoalkanes and cyanide ions?

What are the products?

What is the role of the CN^- ion?

What effect does the reaction have on the carbon chain?

Answer 13

• Heated under reflux in an ethanolic solution of potassium or sodium cyanide
• A nitrile (func group = (-C≡N))
• acts as a nucleophile
• increases its length

Question 14

What are chlorofluorocarbons (CFCs) used for?

Answer 14

reducing the ozone layer and allowing more harmful ultraviolet radiation to reach the Earth’s surface

Question 15

Outline the Sn1 and Sn2 mechanisms of nucleophilic substitution reactions with a halogenoalkane

Answer 15

1. Sn1

• rate determining step - 1st step (slow) - breaking of the C-X bond to form the intermediate cation (induced polarity)
• 2nd step (fast) - nucleophile attacks the carbocation, forming the product

2. Sn2

• the nucleophile performs a backside attack, forcing the leaving group to go, forming the product in 1 step

Question 16

What type of reaction is the reaction between halogenoalkanes and cyanide ions?

Answer 16

Nucleophilic substitution

Question 17

What are the conditions required for the elimination reaction between halogenoalkanes and alkalis?

What are the products?

Answer 17

• alkali must be ethanolic (dissolved in ethanol as a solvent)
• reacts under reflux

2. An alkene and a hydrogen halide, (with a halide salt being a possible byproduct)

Question 18

What are the general conditions required for a substitution reaction, with regards to:

• temp
• OH concentration
• solvent

And what is the main organic product?

Answer 18

• low temp
• dilute OH conc
• water as a solvent

2. An alcohol

Question 19

What are the general conditions required for a elimination reaction, with regards to:

• temp
• OH concentration
• solvent

And what is the main organic product?

Answer 19

1.

• high temp
• high OH concentration
• ethanol as a solvent

2. Alkene

Question 20

What is the term given to halogenolakanes that undergo nucleophilic substitution reactions with KOH, KCN and NH₃?

Answer 20

Unsymmetrical

Question 21

Which of iodine, chloro or bromoalkanes are more susceptible to elimination?

Answer 21

Iodo>bromo>chloro