alkanes

Question 1

what are the physical properties of alkanes

Answer 1

1. insoluble in water - energy released from the weaker pd-induced dipole interactions between polar water and non-polar alkane insufficient to overcome stronger H bonding between water molecules
2. bp increases with number of carbon atoms due to increasing strength of id-id interactions due to increasing number of electrons

• branched has lower mp as it has less SA of contact between molecules meaning weaker id-id int

Question 2

what are alkanes generally unreactive towards

Answer 2

polar reagents

Question 3

why are alkanes unreactive to polar reagents

Answer 3

-relative non-polarity of C-H bond since C and H have similar electronegativities
-electrons are shared equally
-don’t have significant charge to attract charged species.

Question 4

what type of mechanism do alkanes undergo and what are the steps

Answer 4

Free-radical sub
1. initiation
2. propagation
3. termination

Question 5

explain the steps for free-radical sub

Answer 5

1. initiation - UV light provides energy for homolytic cleavage of Cl-Cl bond to form radicals Cl dot (half arrows splitting the bond)
2. propagation - Cl dot consumed in one step and regenerated in another
   - alkane and Cl dot
   -alkane dot and Cl2
3. termination - free-radicals are consumed but not regenerated

Question 6

explain the reactivity of halogens down the group

Answer 6

• down the group they get less reactive
• size of halogen atom increases
• less effective overlap of orbitals

Question 7

how to calculate expected yield

Answer 7

consider the number of positions around each carbon Cl can be subbed into alkane for exp

Question 8

why are the experimental results diff from expected

Answer 8

• stability of alkyl radical
• tertiary>secondary>primary
• rate of substitution of H atom will differ
• tertiary will have the larges amt and react with Cl2 to form pdt

Question 9

explain the stabilities of diff alkyl radicals

Answer 9

• alkyl grps exert electron donating inductive effect
• stabilise a carbon radical by increasing the electron density of the electron deficient C atom
• more alkyl grps means less reactive and more stable

Question 10

explain why iodine cannot be used to make iodoalkanes

Answer 10

• compared to Cl the size of iodine is much larger
• C-I and H-I bonds are weaker due to poorer extent of overlap
• decreasing ease of sub as Hrxn becomes less exo
• rxn not favoured