Topic 10 : Organic Chemistry

• Organic chemistry is the chemistry of carbon compounds • Carbon forms a vast number of compounds because it can form strong covalent bonds with itself • This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known

tendency of identical atoms to form covalent bonds with each other and hence form chains

• A functional group is a specific atom or group of atoms which confer certain physical and chemical properties onto the molecule • Organic molecules are classified by the dominant functional group on the molecule

• Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series • As a homologous series is ascended, the size of the molecule increases This has an effect on the physical properties, such as boiling point and density Assumptions about homologous series • each member has the same functional group • each member has the same general formula • each member has similar chemical properties • members have gradually changing physical properties, for example, boiling point, melting point and density

• The broad trend is that boiling point increases with increased molecular size • Each additional -CH2 adds 8 more electrons to the molecule • This increases the strength of the London Dispersion Forces • Stronger LDF leads to a higher boiling point

• Empirical Formulae • Molecular Formulae • Structural Formulae • Condensed Structural Formulae • Displayed Formulae • Skeletal Formulae

• The empirical formula shows the simplest possible ratio of the atoms in a molecule • For example: Hydrogen peroxide is H2O2 but the empirical formula is HO

• The molecular formula shows the actual number of atoms in a molecule • e.g. C4H10

• The displayed formula shows the spatial arrangement of all the atoms and bonds in a molecule • It shows all of the atoms and the bonds present in an organic compound • The bonds are represented as lines

• Similar to displayed formula but not all bonds are shown • All atoms are still indicated using subscripts but carbon hydrogen bonds are often simplified

• In a condensed structural formulae enough information is shown to make the structure clear, but most of the actual covalent bonds are omitted • Only important bonds are always shown, such as double and triple bonds • Identical groups can be bracketed together • Side groups are also shown using brackets • e.g. CH3CH2CH2CH3

• In a skeletal formula, most hydrogens are omitted and lines represent carbon atoms • Functional groups are still shown and any other atom that isn’t carbon or hydrogen

• Structural isomers are compounds that have the same molecular formula but different structural formulae • Eg. propene and cyclopropane (are both C3H6 but look very different)

• Branch-Chain isomerism • Positional isomerism • Functional group isomerism

• Branch-Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same • This is caused by branching • Eg. pentane and 2,2-dimethylpropane : Both compounds are made up of the same atoms (C5H12) however the longest carbon chain in pentane is 5 and in 2,2-dimethylpropane it is 3 (with two methyl branches)

• Positional isomers arise from differences in the position of a functional group in each isomer • The functional group can be located on different carbons • For example, butan-1-ol and butan-2-ol : Both compounds have an alcohol group and are made up of 4 carbon, 10 hydrogen and one oxygen atom however in butan-1-ol the functional group is located on the first carbon and in butan-2-ol on the second carbon

• When different functional groups result in the same molecular formula, functional group isomers arise • The isomers have very different chemical properties as they have different functional groups • For example, butanol and ethoxyethane : Both compounds have the same molecular formula however butan-1-ol contains an alcohol functional group and ethoxyethane an ether functional group

• Saturated hydrocarbons are hydrocarbons which contain single bonds only resulting in the maximum number of hydrogen atoms in the molecule • e.g. alkanes

• Unsaturated hydrocarbons are hydrocarbons which contain carbon-carbon double or triple bonds • e.g. alkenes or alkynes

• The hydrocarbon side-chain is shown in brackets in the structural formula • The side-chain is named by adding ‘-yl’ to the normal alkane stem • This type of group is called an alkyl group

• Hydrocarbons are compounds containing hydrogen and carbon only • There are four families of hydrocarbons: alkanes, alkenes, alkynes and arenes

• Alkanes have the general molecular formula CnH2n+2. • They contain only single bonds • saturated • The suffix is -ane

• Alkanes are quite unreactive but can burn well. This makes them useful fuels. • They are unreactive due to their strong C-H bonds and C-C covalent bonds which are hard to break apart • They also have a lack of polarity as electronegativities of the carbon and hydrogen atoms in alkanes are almost the same so they share electrons fairly equally - thus they do not react with polar reagents • ONLY REACT IN COMBUSTION AND SUBSTITUTION BY HALOGEN

• Alkenes have the general molecular formula CnH2n • They are said to be unsaturated • Alkenes are named with the suffix -ene • In molecules with a straight chain of 4 or more carbon atoms, the position of the C=C double bond must be specified • The carbon atoms on the straight chain must be numbered, starting with the end closest to the double bond • The lowest-numbered carbon atom participating in the double bond is indicated just before the -ene: • More reactive than alkanes