Alkanes

Question 1

What are some properties of alkanes

Answer 1

Hydrocarbons
Saturated
All atoms bonded via delta bonds - allow free rotation
Tetrahedral shape
109.5 bond angle

Question 2

what are the trends in straight chain boiling points?
What are the reasons for this?

Answer 2

Boiling points increase as the chain gets longer
Induced dipole dipole interactions (london forces) get stronger as number of electrons in molecule increases. More energy needed to overcome intermolecular forces

Question 3

What is the trend in boiling point for branched isomers?
What is the reason for this?

Answer 3

Boiling points decrease as degree of branchin increases
Induced dipole dipole interactions (london forces) get weaker. Branching reduces surface contact between molecules

Question 4

What is the reactivity pattern of the alkanes?
Why?

Answer 4

Generally unreactive
High bond enthalpies in molecules
Very low bond polarity. C and H have similar electronegativities

Question 5

What is the reaction with oxygen?

Answer 5

Combustion
Alkanes combust completely in a plentiful supply of oxygen
CO2 + H2O

Question 6

What is the balanced chemical equation for alkanes as fuels?

Answer 6

C8H18 + 25/2 O2 - 8CO2 + 9H2O

Question 7

What can be produced if incomplete combustion takes place and what are the dangers of this?

Answer 7

Carbon Monoxide gas
Toxic gas that reduces the ability of the blood to carry oxygen

Carbo solid (soot)
Respiritory irritant

Question 8

What are the reaction trends for alkanes reacting with halogens? (Cl2 and Br2)

Answer 8

Alkanes react with halogens in the presence of UV light
Unreactive due to very strong non polar bonds
Reaction proceeds via a radical substitution mechanism

Question 9

What are the steps of the radical substitution mechanism?

Answer 9

Initiation- Cl2 - 2Cl radical (homolytic fission of Cl2)
Propagation- C2H6 + Cl (radical) - C2H5 (Ethile radical) + HCl
C2H5 (radical) + Cl2 - C2H5Cl + Cl
Termination -
Cl + Cl - Cl2 Cl + C2H5 - C2H5Cl C2H5 + C2H5 - C4H10