[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.

MACRO H2 Chemistry Topic 11: Alkanes Notes 4 ©MACRO Academy Chemistry All Rights Reserved under 8613 5830 Copyright Act (Cap. 63) of Singapore FAQ 3 : Predicting the Statistical Ratio of Products • When we only take into account the statistical probability of forming products, there will be 2 products formed from the reaction between propane and chlorine in the presence of UV light. Types of Hydrogen Products Probability six 1o hydrogens two 2o hydrogens 75% 25% • Example 2: Predict the ratio of the products formed when 2-methylbutane reacts with Cl2 under UV light. Types of Hydrogen Products Ratio 3 2 1 6 If substituted, If substituted, If substituted, If substituted, If substituted, If substituted,

MACRO H2 Chemistry Topic 11: Alkanes Notes 5 ©MACRO Academy Chemistry All Rights Reserved under 8613 5830 Copyright Act (Cap. 63) of Singapore FAQ 4 : Using Stability of Radicals to Explain the Discrepancy between Statistical and Actual Ratio • However, when we take into account the stability of the radical intermediates, the observed ratio between the 2 products formed from the reaction between propane and chlorine will be different than the statistical ratio. • Concept: Stability of Radicals Tertiary (3o) Radical Secondary (2o) Radical Primary (1o) Radical Stability Explanation Radical intermediates are unstable (high in energy). The more electron-donating alkyl groups attached to the carbon radical, the more stable the radical and the easier it is to be formed. • Considering the radical intermediates leading to each product, Products Intermediate Statistical Probability Observed Probability 75% 30% 25% 70% Explanation / Answering Technique 1. is formed from a primary radical while is formed from a secondary radical. 2. With more electron-donating alkyl groups attached to the carbon radical, the secondary radical will be stabilized to a greater extent and hence forms at a higher proportion. > >

MACRO H2 Chemistry Topic 11: Alkanes Notes 6 ©MACRO Academy Chemistry All Rights Reserved under 8613 5830 Copyright Act (Cap. 63) of Singapore FAQ 5 : Environmental Consequences of the use of Hydrocarbon (and Others) Pollutant Explanation Effect Unburnt Hydrocarbon Due to incomplete combustion of fuel, carbon monoxide and unburnt hydrocarbon arises Reacts with oxygen, ozone and oxides of nitrogen in the presence of sunlight to form photochemical smog Carbon monoxide Binds irreversibly with haemoglobin in blood, causing drowsiness, headaches, and even fatality. Oxides of Nitrogen At high temperature of combustion (within motor vehicles), N2 and O2 reacts in the following manner: N2 (g) + O2 (g) ➔ 2NO (g) 2NO (g) + O2 (g) ➔ 2NO2 (g) Oxides of nitrogen irritate lungs, causes bronchitis and pneumonia Result in the formation of photochemical smog, which has a harmful effect on plants. NO2 also catalyses the formation of SO3 from SO2, which will dissolve in water to form acid rain. NO2 (g) + SO2 (g) ➔ SO3(g) + NO (g) Catalytic Converter Pollutant How It’s Removed Unburnt Hydrocarbon CxHy(g) + (x + y/4) O2(g) ➔ xCO2(g) + H2O(g) Carbon monoxide 2CO(g) + O2(g) ➔ 2CO2(g) Oxides of Nitrogen 2NO(g) + 2CO(g) ➔ N2(g) + 2CO2(g)

MACRO H2 Chemistry Topic 11: Alkanes Notes 7 ©MACRO Academy Chemistry All Rights Reserved under 8613 5830 Copyright Act (Cap. 63) of Singapore