Alkanes

Question 1

What are some must-know properties of alkanes (physical)?

Answer 1

Boiling points:
Branched chain lower than straight chain due to smaller SA in contact w other alkane molecules, WEAKER (don’t say less extensive) idid interactions

Solubility:
Non polar solvent molecules and alkane molecules form favourable id id interactions, hence miscible. Insoluble in polar solvents (energy released in formation of idid interactions insufficient to overcome… )

Density
Less dense than water. Compare bonding strength (weaker idid vs stronger hydrogen bonds), water molecules pulled more closely to each other, hence alkane molecules per unit volume is less than water molecules per unit volume.

Question 2

What is reactivity of X2 with alkane dependent on? (Which step, what bonds are being broken and formed)

Answer 2

Enthalpy change of Propagation step.
In propagation step,
1. X—X bond broken, H—X bond formed
2. C—H bond broken, C—X bond formed

dependent on strength of H—X bond and C—X bond formed.

The stronger the bond, the more exothermic the reaction, the faster the rate of reaction

Hence, down the group, reactivity decreases. Down the group, HX and CX bonds are longer ans weaker, resulting in an overall less exothermic reaction (formation of the bond releases less energy)

Question 3

Why is there no reaction of alkanes with I2?(CHECK THIS)

Answer 3

H—I bond is so weak (releases so little energy) that overall enthalpy change of the propagation step is endothermic. Product is higher energy than reactant, less energetically stable.

Question 4

[factors affecting rate of formation of different products]—> write out the answering structure for explaining the PREDICTED ratio of formation of 1-chlorobutane and 2-chlorobutane.

(Hint: types of H atoms)

Answer 4

Label carbon atoms as c1-4
The butyl radical leading to the formation of 1-chlorobutane is formed by substituting any of the 6 CHEMICALLY EQUIVALENT H atoms on C1 and C4 (don’t give 2 carbon atoms the same number even if they are essentially symmetrical or wtv), while the butyl radical leading to the formation of 2-chlorobutane is formed by substituting any of the 4 chemically equivalent H atoms on C2 and C3. Hence, ratio of products is 6:4 ie 3:2

Question 5

[factors affecting formation of major and minor alkanes—FRS]
Explain why the actual ratio is diff from predicted ratio for 1-chlorobutane and 2-chlorobutane (just explain the actual ratio)

Answer 5

Secondary carbon radical leading to the formation of 2 chloro-butane is more stable than the primary carbon radical leading to the formation of 1-chlorobutane, as secondary carbon radical has more electron donating alkyl groups bonded to the carbon than the primary radical, which increases the electron density on the carbon with the unpaired electron. Hence, this increases the rate of reaction, increasing proportion of 2-chlorobutane formed in the product mixture.

Question 6

Applications of alkanes topic
Write out reactions that pollutants produced from internal combustion engines undergo in a catalytic converter

Question 7

List the negative effects of the different pollutants released from internal combustion engines

Question 8

Why are alkanes unreactive?

Answer 8

Fully saturated sp3 hybridised carbons
Strong and non polar C-H bonds
Lack of electron rich or deficient sites on molecule makes it unreactive to polar reagents