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11.1 Homologous series

11.1.1

A 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 C_nH_2n+2.

11.1.2

The 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)

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11.2 Hydrocarbons

11.2.1

Basically, each will have a CH₃ group at either end (except methane has only one CH₄) and fill out the required number of CH₂ groups.

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11.2.2

Names:

• Methane,
• ethane,
• propane,
• butane,
• pentane,
• hexane.

11.2.3

Basically, move the groups around to make branches, C₁ should have 1, C₂ has 1, C₃ has 1, C₄ has 2, C₅ has 3.

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11.2.4

These structures will be the same as 11.2.1, except two hydrogens on adjacent carbons are replaced by a double bond between those hydrogens.

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11.2.5

Complete 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.

11.2.6

The 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 groups

11.3.1 : Functional groups

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• Alkanal - RCHO (with a double bonded O coming off the C (aldehyde)...but it can be moved along the chain -> keytone). Naming - (aldehyde) end in al - ie ethanal. (keytone) end in one - ie enthanone.

• Alkanoic acid - R-COOH Naming - end in oic acid - ie ethanoic acid (commonly called carboxilic acid)

• Alkanol - R-OH. Naming - end in ol - ie ethanol.

• Amide - RCOONH2 Naming - end in amide - ie ethylamide.

• Amine - R-NH2 The two hydrogens on the N can be replaced by R groups to give primary, secondary and tertiary structures). Naming - end in amine - ie ethylamine.

• Ester - R-COO-OR. Naming...Think of it like an alkanoic acid with a carbon chain rather than a H...the alkanoic acid type bit is ...oate this is preceded by the stem of the other half - ie Ethyl ethanoate.

• Halogenoalkane - R-X. Naming - name of halogen (fluro, chloro, bromo, iodo) followed by R name - ie Chloroethane.

11.3.2

Functional groups can actually be isomers (though their properties are not generally similar). For example ethanoic acid and methyl methanoate are isomers (CH₃COOH vs HCOOCH₃).

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11.3.3

Optical 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.

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11.3.4

O-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).

11.3.5

Reactions 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.

- Hydrogen adds to the double bond

- bromine adds onto the double bond

- HBr adds across the double bond

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11.3.6

(addition) Polymerisation of alkenes (by a free radical mechanism)

Initiation:

(R° is a free radical with a lone electron)

Propagation:

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Termination:

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Polythene:

• Monomer is CH₂=CH₂

• General polymer is -[-CH₂-CH₂-]n-

Polyvinal chloride:

• Monomer is CClH=CH₂ (chloroethene)
• General polymer is -[-CH₂-CHCl-]n-

11.3.7

Production of an ester from an alkanol and alkanoic acid...this is an addition elimination reaction (or addition-dehydration, since we're eliminating water)

Esters are commonly used as artificial flavoring agents ... Mmmmmm ... Ester ...

11.3.8

Oxidation 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.

[Unparseable or potentially dangerous latex formula. Error 6 ].

Ethanal is an intermediate which is intentionally not isolated so it can be oxidized again.

11.3.9

The 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).

11.3.10

Condensation polymers

Nylon - hexane -1,6-diamine + hexanedioic acid

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As each new group is created, a water molecule is eliminated.

Polyester - benzene-1,4-dicarboxylic acid + ethane-1,2-diol

(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.

11.3.11

This reaction produces a series of stronger and stronger nucleophiles until a complex ion, (CH3CH2)4N+ is produced.

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11.3.12

Formation of peptides and proteins to form 2-amino acids

Amino Acids - - carbon atom (asymmetric) is connected to 4 different species -> optically active (except glycene).

(poly peptide) This group can join to other peptides to form a protein.

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