IGCSE Chemistry 0620: Alkane Substitution with Chlorine. Evaluating Claims

A student mixes gaseous pentane, $C_5H_{12}$ , with chlorine gas in a clear glass flask and exposes the flask to strong sunlight. Over several minutes, the pale green colour of the chlorine fades, and a colourless, acidic gas escapes from the flask.

The student writes three conclusions in their notebook.

"This must be an addition reaction, because a chlorine atom has added onto the pentane molecule."
"Sunlight only speeds the reaction up. The same reaction would still happen at roughly the same rate if the flask were instead left in a dark cupboard at the same temperature."
"Since monosubstitution occurs and one mole of pentane reacts with one mole of chlorine, the organic product must have the molecular formula $C_5H_{11}Cl$ ."

(a) State, with a reason, whether conclusion 1 is correct. [2]

(b) State, with a reason, whether conclusion 2 is correct. [2]

(d) State what provides the energy needed to break the Cl–Cl bond and start this reaction, and name the type of bond that is broken. [2]

Part (a): Is conclusion 1 correct?

Conclusion 1 is incorrect.

In this reaction, a hydrogen atom is removed from a pentane molecule at the same moment that a chlorine atom is attached in its place, and the removed hydrogen leaves as part of a molecule of hydrogen chloride, HCl. Replacing one atom (or group of atoms) with a different atom (or group) is the definition of a substitution reaction, not an addition reaction.

An addition reaction would simply add extra atoms across a carbon–carbon double bond with nothing removed and only one product formed. Pentane is a saturated alkane with no C=C double bond, so it is not even capable of undergoing addition.

Part (b): Is conclusion 2 correct?

Conclusion 2 is incorrect.

This reaction is described as photochemical: light energy (from sunlight or ultraviolet light) is needed to break the Cl–Cl bond and provide the activation energy that starts the reaction. In the dark at room temperature, there is no equivalent source of activation energy available, so essentially no reaction takes place. Light is not just making an already-occurring reaction faster, it is what allows the reaction to happen at all.

Part (c): Is conclusion 3 correct?

Conclusion 3 is correct.

Pentane has molecular formula $C_5H_{12}$ . In monosubstitution, exactly one hydrogen atom is replaced by one chlorine atom: $C_5H_{12} \rightarrow C_5H_{11}Cl$

So the organic product does have the molecular formula $C_5H_{11}Cl$ , as the student’s third conclusion states.

Part (d): What breaks the Cl–Cl bond?

The light energy supplied by the sunlight (ultraviolet light) provides the activation energy needed to break the bond.

The bond broken is the covalent bond between the two chlorine atoms in a $Cl_2$ molecule.

Final answers

(a) Incorrect. This is a substitution reaction (H replaced by Cl, HCl released), not addition.
(b) Incorrect, sunlight provides the activation energy needed to start the reaction; without it, the reaction barely occurs.
(c) Correct. The product is $C_5H_{11}Cl$ .
(d) Light (ultraviolet light) provides the activation energy; the bond broken is the Cl–Cl covalent bond.