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Revision:Organic Chemistry - 11TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Organic Chemistry - 11 11.1 Homologous series11.1.1A homologous series is a set of compounds whose components differ by a single repeating functional group. In the case of (straight chain) alkanes, CH2, and their general formula is CnH2n+2.
11.1.2The boiling points of alkanes increase as the chains get longer (increased number of electrons --> increased van de Waal's forces), increasing rapidly initially but flattening off (since the number of additional CH2 units required to double the chain length increases rapidly, so it flattens off - or you could just believe it)
11.2 Hydrocarbons11.2.1Basically, each will have a CH3 group at either end (except methane has only one CH4) and fill out the required number of CH2 groups.
11.2.2Names:
11.2.3Basically, move the groups around to make branches, C1 should have 1, C2 has 1, C3 has 1, C4 has 2, C5 has 3.
11.2.4These structures will be the same as 11.2.1, except two hydrogens on adjacent carbons are replaced by a double bond between those hydrogens.
11.2.5Complete combustion produces CO2 and H2O, incomplete combustion produces CO, C and H2O (usually occurs with saturated alkanes, where there is a lot of hydrogen, or where there is a limited supply of oxygen). C produces a 'dirty' flame leaving carbon deposits on everything, CO is toxic and CO2 is a greenhouse gas. Incomplete combustion is where the carbon is not completely oxidised.
11.2.6The combustion of hydrocarbons is an exothermic process (otherwise there wouldn't be much point in burning them would there...). This is a result of the fact that the O-H bond is stronger than the C-H bond, and the C=O bond is stronger than the C-C. This means that, the C-C and C-H bonds breaking requires energy, but this is more than made up for by the energy released by the formation of the C=O and O-H bonds.
11.3 Other functional groups11.3.1 : Functional groups(missing image/diagram)
11.3.2Functional groups can actually be isomers (though their properties are not generally similar). For example ethanoic acid and methyl methanoate are isomers (CH3COOH vs HCOOCH3).
11.3.3Optical isomers result if a carbon atom has 4 different groups on each bond. If this is the case, the compound exists in 2 entantiomeric forms (ie optical isomers). In general they react very similarly except in the presence of other optical isomers (also known a chiral molecules -- the chiral center is the carbon atom with 4 different groups). The two enantiomers are mirror images of each other which cannot be superimposed on each other. Biological systems commonly have a strong preference for one enantiomer over the other ( one can be bitter, the other sweet for example ). The isomers can be identified by their effect on polarised light(by a polarimeter)...when polarised light passes through one isomer it will be rotated to the left, while the other will rotate to the right.
11.3.4O-H groups create hydrogen bonding (alcohols, alkanoic acids) -> less volatile and also solubility (long chain molecules become less soluble since the non-polar chain dominates the molecule). C=O bonds in Alkanoic acids, Alkanals -> polar bond...dipole forces...higher bp (more significant in small molecules). Small molecules are soluble due to polarity (effect decreases with long chains). Esters...no Hydrogen bonding -> very volatile, low BP. Polar molecules, therefore short are soluble in water. Amides...N-H bond is polar with extensive hydrogen bonding -> highly soluble and all molecules have higher BP than alkanes. Amines...Hydrogen bonding present in Primary and secondary -> soluble (when short) and higher than alkane boiling points. Tertiary amines are very similar to alkanals (but branched -> less dense packing etc). Halogenoalkanes...short molecules will be soluble due to polar bonds, BP will be somewhat higher Acid Base properties...Alkanoic (carboxilic) acids are, obviously, acidic. Alcohols are generally not due to the donating effect of the R group. Amines are derivatives of ammonia, and so are basic (though stronger due to the donating effect of the R groups). Amides are not due to withdrawing effect of the C=O group. The others, in general, are neutral (however alkanols can, in acidic conditions act as a base and accept a proton, though the electron donating effect of the alkyl groups generally stop any acidic action).
11.3.5Reactions of alkenes with stuff...hydrogen, bromine, hydrogen halides and water. Before we begin : The C=C bond is not twice as strong as a C-C bond...the second is weaker making it easier to break, and thus a reactive site...this reactivity makes alkenes important starting molecules in the production of other organic molecules.
11.3.6(addition) Polymerisation of alkenes (by a free radical mechanism) Initiation:
Polythene:
Polyvinal chloride:
11.3.7Production of an ester from an alkanol and alkanoic acid...this is an addition elimination reaction (or addition-dehydration, since we're eliminating water)
11.3.8Oxidation of Ethanol to ethanoic acid: This process requires a primary alcohol (which ethanol is) otherwise the reaction is stopped because the intermediate formed is a keytone rather than an aldehyde.
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\mathsf{CH_3CH_2OH ---{}_{oxidized by K_2Cr_2O_7/H^+} --> CH_3CHO ---{}_{oxidized by K_2Cr_2O_7/H^+ ---> CH_3COOH}
11.3.9The reaction is between ethanoic acid and ethanamide to form N-ethyl enthanamide (the N means the ethyl group is connected to the N atom. It is also a dehydration reaction (ie water is eliminated).
11.3.10Condensation polymers Nylon - hexane -1,6-diamine + hexanedioic acid
As each new group is created, a water molecule is eliminated.
(once again, water is eliminated each time) Notice the fact that two functional groups are required on each monomer. Otherwise the reaction would stop without producing a long chain.
11.3.11This reaction produces a series of stronger and stronger nucleophiles until a complex ion, (CH3CH2)4N+ is produced.
11.3.12Formation of peptides and proteins to form 2-amino acids Amino Acids -
(poly peptide) This group can join to other peptides to form a protein.
CommentsInteresting terminology used in the article (e.g. 'Alkanal', 'Alkanoic acid' and 'Alkanol'). Be very careful about using the correct language depending on which board you are studying. You may not gain full marks (one examiner comments that 'we don't mark correct chemistry as incorrect, but we award marks following specific mark schemes'). The above, for all UK exam boards, should be 'aldehyde', 'carboxylic acid' and 'alcohol'. |
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