Alkanes

Question 1

General formula

Answer 1

C_nH_2n+2

Question 2

First 4 in homologous series

Answer 2

1. Methane
2. Ethane
3. Propane
4. Butane

Question 3

Physical properties

1. Interactions + Reason

Answer 3

1. C-H bond non-polar (Difference in EN between C and H atom negligible) => IMFOA - id-id interactions

Question 4

Physical properties

How does B.P/M.P change with
1. Increasing no. of C atoms
2. Increasing degree of branching

Answer 4

1. B.P/M.P increases with more C atoms
   => no. of C atoms increases
   => no. of electrons per molecule increases
   => size of electron cloud increases
   => ease of polarisation of electron cloud increases
   => strength of id-id interactions increases
   => more E required to overcome
2. B.P/M.P decreases with increasing degree of branching
   => Increasing degree of branching
   => molecule more spherical
   => SA avail for intermolecular interactions decreases
   => extent of contact between neighbouring molecules decreases
   => strength of id-id interactions decreases
   => less E required to overcome

MUST LINK TO ENERGY REQUIRED

Question 5

Physical properties

Alkanes Solubility in
1. Non- polar solvents
2. Polar solvents

Answer 5

Alkanes are NON-POLAR

SOLUBLE in NON-POLAR solvents (benzene)
INSOLUBLE in POLAR solvents (water)

Key concept: solubility => similarity of interaction

Question 6

3 ways to prepare Alkanes

Answer 6

1. From Alkenes
   - Reduction (Ni, heat)
2. From Alkynes
   - Reduction (Pd, heat)
3. From Benzene
   - Reduction (Ni catalyst, High T&P)

Question 7

Reactions of Alkanes

Answer 7

1. Combustion
2. Free radical substitution (Halogenation)
   - substitution of 1 or more H atoms of alkane by halogen atoms

Question 8

Free radical substitution RNC

Answer 8

X₂ , UV light (l) or (g)

=> NO (aq)

Question 9

Definition: Homolytic fission

Answer 9

Covalent bond breaks such that each of the atoms involved in forming the bond acquires 1 of the 2 bonding electrons, forming free radicals

Question 10

2 reasons for the UNREACTIVITY of Alkanes

Answer 10

1. NON-POLAR
   - Alkanes do NOT contain any region of high electron density
   => does NOT attract electrophiles
   - Alkanes do NOT contain any electron deficient sites
   => does NOT attract nucleophiles
2. RELATIVELY STRONG C-H & C-C bonds
   - These bonds do NOT break under normal conditions