alkanes

Question 1

formation of a σ bond

Answer 1

direct single overlap of orbitals between bonding atoms
occurs in C-H and C-C
allows free rotation about the σ bond

Question 2

trend in boiling point of alkanes

Answer 2

as the C chain length increases, boiling point increases
there is more surface contact between molecules and more induced dipole-dipole interactions between molecules which need more energy to overcome
as branching increases, boiling point decreases
there is less surface contact between molecules and fewer induced dipole-dipole interactions between molecules which need less energy to overcome

Question 3

combustion of alkanes

Answer 3

in plentiful oxygen, alkanes undergo complete combustion to form CO2 and H2O
in limited oxygen, alkanes undergo incomplete combustion to form CO and H2O

Question 4

radical

Answer 4

a species with an unpaired electron

Question 5

free radical substitution of alkanes

Answer 5

reagents = halogen and excess alkane
conditions = UV radiation or 300oC

Question 6

limitations of free radical substitution

Answer 6

leads to formation of a mixture of products by further substitution
reactions at different positions in carbon chain creates structural isomers
low % yield

Question 7

how to prevent further substitution

Answer 7

add excess alkane

Question 8

free radical substitution mechanism for methane and chlorine

Answer 8

initiation:
Cl2 -> 2Cl*
propagation:
CH4 + Cl* -> CH3 + HCl
CH3 + Cl2 -> CH3Cl + Cl
termination:
Cl + Cl* -> Cl2
Cl* + *CH3 -> CH3Cl
*CH3 + *CH3 -> C2H6