[SHP] Alkane Notes

MACRO H2 Chemistry Topic 11: Alkanes Notes 2 ©MACRO Academy Chemistry All Rights Reserved under 8613 5830 Copyright Act (Cap. 63) of Singapore Checklist: Alkanes Check (✓) 1. I can recall the reactions an alkane can undergo, as well as their reagents and conditions 2. I can draw and describe the mechanism of free radical substitution, with particular reference to the initiation, propagation and termination reactions 3. I can explain the general un-reactivity of alkanes towards polar reagents 4. I am able to predict the possible products formed during free radical substitution, as well as their statistical probability ratio. 5. I can explain, using the concept of stability of radicals, why isomers of alkyl halides are formed in different proportions during free radical substitution 6. I able to recognise the environmental consequences of: a. carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the internal combustion engine and of their catalytic removal b. gases that contribute to the enhanced greenhouse effect 7. I am able to recognise that petroleum, a chemical feedstock, is a finite resource and the importance of recycling

MACRO H2 Chemistry Topic 11: Alkanes Notes 3 ©MACRO Academy Chemistry All Rights Reserved under 8613 5830 Copyright Act (Cap. 63) of Singapore FAQ 1 : Why are alkanes generally unreactive towards polar reagents? • Due to the similarities in electronegativities of the C and H atoms, C-C and C-H bonds are non- polar. • C-C and C-H bonds are strong as well (B.E (C-C) = 350 kJ mol-1 ;B.E (C-H) = 410 kJ mol-1 • Hence, alkane molecules do not contain any significant amount of positive or negative charge and are unreactive towards polar reagents. FAQ 2 : Describing the Mechanism of Free Radical Substitution • Consider the mechanism when propane is reacted with chlorine in the presence of UV light to form 1-chloropropane. Remarks Initiation: B.E (Cl-Cl) = 244 kJ mol-1 < B.E (C-H) = 410 kJ mol-1 Therefore, C-H bonds are not affected during the initiation step. Propagation: Propagation steps always produce a radical to further propagate the reaction (The Cl· radical generated in the second step can be used to react with another propane molecule) Termination: Termination steps consume radical and forms stable products, thereby termination the chain reaction.

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