halogenoalkanes

Question 1

what are the physical properties of halogenoalkanes?
bp/mp solubility

Answer 1

• higher bp and mp than alkanes of the same number of C atoms due to the presence of pd-pd interactions between polar RX molecules and greater no. of electrons
• insoluble due to inability to form H bonds with water

Question 2

how to prepare halogenoalkanes

Answer 2

• from alkenes
  HX at rtp
• from alcohols
  PX3, SOCl2 and HX and heat under reflux or PCl5 at rtp

Question 3

what type of reaction do halogenoalkanes undergo

Answer 3

Nucleophilic sub - C-X bond is polar due to the highly electronegative halogen atom meaning that there is a very electron deficient C atom susceptible to nucleophiles

Elimination - forms alkenes

Question 4

what can halogenoalkanes form

Answer 4

Nucleophilic sub
1. Form alcohol
NaOH or KOH with heat under reflux

2. Form nitriles
   alc KCN with heat under reflux
3. Form amine
   alc NH3 in excess heated under pressure in a sealed tube

Elimination to form alkene
alc NaOH or KOH with heat under reflux

Question 5

what is a special consideration for gem-dihalides or gem-trihalides

Answer 5

product formed is either a carbonyl compound of carboxylic acid - e.g cl bonds are replaced by OH but form an unstable structure so it has to lose a water molecule

Question 6

what is the distinguishing test for RX

Answer 6

1. Heat RX with NaOH (aq)
2. Acidify with dilute HNO3 (aq)
3. Add AgNO3 (aq)

white - AgCl
cream - AgBr
yellow - AgI

Question 7

what is the effect of halogen atom on reactivity

Answer 7

Larger halogen atom means longer C-X bond which means weaker C-X strength and the bond is more easily broken

Question 8

what type of reaction do primary halogenoalkanes undergo

Answer 8

• SN2 where the nucleophilic OH- approaches the carbon deficient atom form the side opp to the bromine atom (show tetrahedral nature of halogenoalkane and arrow form lp of electrons to carbon deficient atom)
• pair of electrons in C-Br bond is completely acquired by Br leading to formation of Br- ( show partial charges and arrow from C-Br bond to Br)
• product inverted from starting molecule

Question 9

what is the rate of reaction given by SN2

Answer 9

rate = k(RX)(OH-)

Question 10

what type of reaction do tertiary halogenoalkanes undergo

Answer 10

• SN1 where tertiary halogenoalkane undergoes heterolytic fission to form halide ion cand carbocation ( arrow from C-X bond to X showing partial charges) this is also the slow step
• Carbocation is readily attack by nucleophiles to form product ( show arrow from lp of OH- to the electron deficient C) this is also the fast step

Question 11

why do SN1 products not rotate plane polarised light

Answer 11

Carbonium ion has a trigonal planar arrangement around C with a positive charge meaning there is an equal chance of attack from either side forming a racemic mixture

Question 12

what is the rate of reaction given by SN1

Answer 12

rate = k(RX)

Question 13

what are the factors that favour SN1 and SN2 mechanism

Answer 13

SN1 - stability of carbocation
higher stability = more favoured

SN2 - steric hinderance
less steric hinderance means easier approach of nucleophile

Question 14

why is chlorobenzene less reactive than RX

Answer 14

• p orbital of halogen atom overlaps with p orbital of C atom in the benzene ring
• delocalisation of lp into the benzene ring
• confers partial double bond character, strengthening the bond
• harder to replace

similar for cl bonded to a double bond

Question 15

what is a reaction chlorobenzene undergoes but benzene and RCl dont

Answer 15

hydolysis with NaOH(aq) at 360 degrees under pressure to form carboxylic salt