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Key points for OCR AS chemistry Unit 2 (ideal for flash cards, podcasts or just general
1. Representing organic compounds
· The empirical formula is the simplest whole-number ratio of atoms of each element present in a
· The molecular formula is the actual number of atoms of each element in a molecule.
· The general formula is an algebraic formula for a member of a homologous series, for example,
CnH2n+2 for an alkane.
· Structural, displayed and skeletal formulae are all used to show organic molecules in different
· Structural isomers are compounds with the same molecular formula, but different structural
· Stereoisomers are compounds with the same structural formula, but with a different arrangement
· E/Z isomerism is an example of stereoisomerism in which lack of free rotation about the C=C bond
allows two different forms to exist.
· Cis-trans isomerism occurs when two hydrogen atoms, on different carbon atoms, adopt either the
same or opposite sides of the C=C.
3. Hydrocarbons from crude oil
· A hydrocarbon is a compound containing hydrogen and carbon atoms only.
· Crude oil is separated by fractional distillation, in which the fractions have different boiling points.
· The carbon atom in hydrocarbons, as in alkanes, is forming four single covalent bonds in a
· As the chain length of the hydrocarbons increases, so too does the size of the van der Waals' force,
resulting in a high boiling point.
4. Reaction of alkanes
· Alkanes are hydrocarbons, and their molecules are weakly attracted by van der Waals' forces.
· Alkanes are saturated hydrocarbons; this means that they consist of only carbon single bonds.
· Alkanes are mainly used as fuels, when combusted they provide heat energy.
· Alkanes react with halogens in ultraviolet light to form halogenoalkanes. This is a free radical
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· Alkenes are unsaturated hydrocarbons; they consist of one or more carbon double bonds, making
them more reactive than alkanes.
· A double bond consists of a sigma and a pi bond. The pi bond is formed by the adjacent overlap of
· The pi bond is the reactive part of the double bond; the pi bond is of lower reactivity.
· The arrangement of the bond about the carbon double bond is trigonal planar, with an internal
angle of 120°.