Halogenoalkanes

Question 1

Name the halogens

Answer 1

Fluorine, chlorine, bromine and iodine

Question 2

What makes up an halogenoalkane

Answer 2

Alkane skeleton with one or more halogen

Question 3

How to name halogenoalkanes

Answer 3

Prefixes: Fluoro, chloro, bromo, iodo = which halogen present
Numbers used to show which carbon the halogen bonded to
Di, tri, tetra … used to show how many atoms of each halogen present
Halogens listed in alphabetical order

Question 4

Polarity of halogenoalkanes

Answer 4

• Halogens F, Cl, Br and I have higher electronegativities than C
• Covalent bonds between carbon and halogens are polar (electrons not evenly distributed)
• The more polar the greater the dipole (I, Br, Cl, F in order)

Question 5

Stereoisomers

Answer 5

= isomers whose atoms are connected in the same order but have different spatial arrangements

Question 6

What is a chiral structure

Answer 6

Compound that has at least 1 carbon bound to at least 4 other atoms/groups
Are mirror objects but cant be superimposed

• chiral carbon atom = asymmetrical carbon/ stereo centre

Question 7

What is an enantiomer

Answer 7

A pair of chiral molecules

Question 8

What is an achiral structure

Answer 8

Has a mirror and can be super imposed

Question 9

What is racemate

Answer 9

• a 1:1 mixture of enantiomers (left handed : right handed)
• many drugs are chiral with 1 enantiomer possessing desired properties
• is starting materials achiral and products chiral it is a racemic mixture
• opposite enantiomer may be biologically inactive, active in a different way or produce undesired effects e.g. thalidomide

Question 10

What are the different ways to describe enantiomers

Answer 10

• draw it
• use R and S system
• Use +/- system
• use d/l system

Question 11

Explain how to use the R and S system

Answer 11

• way to describe enantiomers
  1. Identify the stereogenic centre
  2. Assign priority number to each group
• atoms with higher atomic number get higher priority
• if 2 atoms are the same then assess the atoms attached to them
  3. Arrange the molecule so that the lowest priority group is pointing away
  4. Follow 1,2,3 around the circle
• clockwise = R
• anti clockwise wise = S

Question 12

Explain how to use the d/l and +/- minus systems

Answer 12

Used to describe enantiomers and detect their differences

• polarised light is rotated in different directions by enantiomers
• clockwise = d or +
• anti clockwise l or -

Question 13

What are diastereoisomers

Answer 13

Stereoisomers that dont have mirror images

Question 14

What are the 2 different reactions of halogenoalkanes

Answer 14

• nucleophilic substitution

- elimination

Question 15

What is a nucleophile

Answer 15

Chemical species that donates a pair of electrons

Question 16

What are the 2 mechanisms within nucleophilic substitution

Answer 16

Sn1 and Sn2

Question 17

Describe the Sn1 mechanism

Answer 17

• leaving group goes first and then the nucleophile joins later (heterolytic dissociation)
• atom leaves halogenoalkane = unstable
• nucleophile then attacks and binds
  2 transition states
  Rate of reaction depends on the first slow step = reaction rate is 1st order
  Nucleophile can join at the top or bottom and will become a different form of stereoisomer

Question 18

Describe the Sn2 mechanism

Answer 18

• leaving group goes and nucleophile joins at the same time
• only 1 transition state
• rate of reaction depends on nucleophile and carbon skeleton = rate of reaction 2nd order
• Walden inversion takes place

Question 19

Describe elimination reaction

Answer 19

• nothing replaces the lost atom
• an alkene is formed
• the nucleophile attacks the H instead of the C

E1 = 2 step reaction
E2 = 1 step reaction

Question 20

When is Sn1 used and when is Sn2 used?

Answer 20

It depends on the hyperconjugation of the carbon skeleton

• tertiary is the most stable = SN1 reaction
• primary = SN2 reaction

Question 21

When is elimination used and when is substitution used?

Answer 21

• depends on the nucleophile and how strong it is
• strong = elimination
• weak = substitution