RI 2022 Alkanes v2.0

• Content • Alkanes (exemplified by ethane) RAFFLES INSTITUTION YEAR 5 H2 CHEMISTRY 2022 Lecture Notes 10: Alkanes (i) free-radical substitution reactions • Hydrocarbons as fuels Learning Outcomes Candidates should be able to: (a) explain the general unreactivity of alkanes, including towards polar reagents (b) describe the chemistry of alkanes as exemplified by the following reactions of ethane: (i) combustion (ii) free-radical substitution by chlorine and by bromine (c) describe the mechanism of free-radical substitution with particular reference to the initiation, propagation and termination reactions (d) recognise the environmental consequences of: (i) carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the internal combustion engine and of their catalytic removal (ii) gases that contribute to the enhanced greenhouse effect (e) recognise that petroleum, a chemical feedstock, is a finite resource and the importance of recycling Lecture Outline 1 2 3 4 5 6 7 8 Introduction Isomerism in alkanes Physical properties Preparation of alkanes Reactions of alkanes Free-radical substitution Alkanes as fuels Summary References • Chemistry in Context (by Graham Hill & John Holman) • Cambridge International AS and A Level Chemistry (by Peter Cann & Peter Hughes) • Website: www.chemguide.co.uk aliphatic I organic compounds I hydrocarbons halogen derivatives hydroxy compounds carbonyl compounds aromatic carboxylic acids & derivatives nitrogen compounds saturated unsaturated .----I.__, .--__._I -.,.-__, I I I open-chain alicyclic alkenes alkynes arenes (acyclic or (ring) non-?clic) \ alkanes cycloalkanes CH CnH2n n 2n+2 -1-

_ 1---Ll_ln_tr_o_d_uc_t_:lo_n _____ _______________ ___ • The alkanes are the simplest of the homologous series found in organic chemistry. • Alkanes are hydrocarbons since they contain only carbon and hydrogen. Other hydrocarbons include alkenes, alkynes and arenes. • Alkanes are saturated hydrocarbons because they contain only carbon-carbon and carbon-hydrogen single bonds and do not contain any double or triple bonds. Unsaturated hydrocarbons such as alkenes and alkynes contain multiple bonds (either double or triple bonds) between carbon atoms. • The open-chain alkanes form a homologous series of saturated hydrocarbons with. the general molecular formula, CnH2n+2. Successive members differ in composition by one CH2 group (methylene group). • Each C atom in an alkane molecule is sp3 hybridised and has a tetrahedral molecular geometry around it. The bond angle around each C atom (in an open-chain alkane) is approximately 109.5°. • Cycloalkanes are alkanes that contain one or more rings of carbon atoms. They are saturated hydrocarbons in which the carbon atoms are arranged to form rings. • Cycloalkanes with only one ring have the same general molecular formula (i.e. CnH2n) as the alkenes with only one C=C bond. Tip: For an