alkanes

Question 1

simple characteristics of alkanes (shape, hybridised, ? sigma bonds formed, formula)

Answer 1

-tetrahedral in shape, sp3 hybridised, forms 4 sigma bonds using its 4 hybrid sp3 orbitals
-saturated hydrocarbons CnH2n+2

Question 2

branched alkanes

Answer 2

can branch (>3 C atom) so can have constitutional (chain) isomerism

Question 2

cycloalkanes

Answer 2

-ring of C atoms
-CnH2n
-similar chem to straight and branched ones

-cyclopropane and cyclobutane are unstable molecules due to ring STRAIN(occurs when carbons are forced to be in bond angle of 60°/90°, instead of 109.5°)

Question 3

Why bp of straight chain alkanes increases as no. of C atoms increases?

Answer 3

increase in e cloud size = more polarisable (tendency of substance to form dipole moment) = stronger id-id = more energy to overcome these forces

Question 4

Why branched chain alkanes has a lower bp than straight ones?

Answer 4

branched -smaller surface area of contact = weaker id-id = lesser energy needed to overcome these forces

Question 5

Why alkanes are soluble in non-polar solvents?

Answer 5

idid between alkane and non-polar solvents are similar in strength compared to those within them = sufficient energy released to overcome idid within them

Question 6

why are most alkanes less dense than water?

Answer 6

idid in alkanes are weaker than H-bonding between H2O molecules
alkane molecules pulled less closely compared to h2o molecules

density = mass/volume

Question 7

why alkanes are generally unreactive?

Answer 7

-C atoms are fully saturated cuz sp3 hybridised
-strong & non-polar nature of C-H bonds
- lack of e-rich and e-deficient sites = unreactive towards polar reagents

Question 8

combustion in alkanes

Answer 8

complete: highly exo, CO2 + H2O

incomplete: highly exo, CO + H2O

Question 9

free radical substitution mechanism

Answer 9

1. Initiation: Cl-Cl -(UV LIGHT)- > 2Cl・
2. Propagation:
   Cl・+ H-R -> HCL + R・
   R・ + Cl-Cl ->RCl +Cl・(regenerated, meaning it can react with another molecule, self-sustaining cycle, chain reaction)
3. Termination:
   Cl・+ Cl・-> Cl2
   ・R + Cl・ -> RCl
   ・R +・R -> RR

Question 10

how to predict the mixture of mono-substituted products?

Answer 10

1. Assuming all H atoms undergo substitution at the same rate, can predict by comparing NO. OF EQUIVALENT H ATOMS that give rise to each products

Ans: When any Ha atoms are substituted, A is formed. When any Hb atoms are substituted, B is formed. Hence ratio of A:B in product mixture is expected to be :

2. ALSO, rate of substitution of H atoms affect the mixture(if qn didn’t say then we don’t take this into consideration. this is the reason why actual product ratio differs from expected ratio)

In increasing stability,
eg. methyl < primary < secondary < tertiary

formation of more stable carbon radical will increase the rate of reaction = increase the proportion of the isomers formed from this radical

Question 11

environmental consequences for CO, oxides of N, unburnt hydrocarbons arising from INTERNAL COMBUSTION ENGINES

Answer 11

CO: CO + haemoglobin in blood -> carboxy-haemoglobin (prevents transportation of O2 to body)

oxides of N: forms acid rain = corrode buildings, destroy crops and marine life. the gas itself causes respiratory problems

unburnt hydrocarbons: in strong sunlight, becomes photochemical smog = lung damage

Question 12

what does a catalytic converter do

Answer 12

1. oxidies CO
2. completely combust hydrocarbons
3. covert oxides of N to N2(g) and O2(g)

Question 13

CO2, CH4, N2O contribution to greenhouse effect and its origin

Answer 13

overall these greenhouse gases trap heat in the atmosphere.

origins of these greenhouses gases:
CO2: burning of fossil fuels
CH4: livestock and decay of organic waste
N2O: agricultural and industrial activities

Question 14

Racemic mixture

Answer 14

There are equal chance for the Cl. / Cl2 to attack the free radical from both sides of the plane forming equal proportion of the 2 enantiomers