Topic 11 - Organic chemistry

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[edit] 11.1 Homologous series

[edit] 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, their general formula is C_nH_2n+2.

[edit] 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.

(This is because the number of additional CH₂ units required to double the chain length increases rapidly…so it flattens off. Or you could just believe it.)

[edit] 11.2 Hydrocarbons

[edit] 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.

[edit] 11.2.2

Names: Methane, ethane, propane, butane, pentane, hexane.

[edit] 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.

[edit] 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.

[edit] 11.2.5

Complete combustion produces CO₂ and H₂O, incomplete combustion produces CO, C and H₂O (incomplete combustion usually occurs with unsaturated 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.

[edit] 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.

[edit] 11.3 Other functional groups

[edit] 11.3.1 : Functional groups

Alkanal: RCHO (with a double bonded O coming off the C (aldehyde), but it can be moved along the chain -> ketone).
Naming: (aldehyde) end in al – i.e. ethanal. (ketone) end in one – i.e. enthanone.

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

Alkanol: R-OH.
Naming: end in ol – i.e. ethanol.

Amide: RCOONH₂
Naming: end in amide – i.e. 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 – i.e. ethylamine.

Ester: R-COO-R.
Naming: Think of it like an alkanoic acid with a carbon chain rather than a H. The alkanoic acid type bit is …oate which is preceded by the stem of the other half – i.e. Ethyl ethanoate.

Halogenoalkane: R-X.
Naming: Name of halogen (fluro, chloro, bromo, iodo) followed by R name – i.e. Chloroethane.

[edit] 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).

[edit] 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 (i.e. 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.

[edit] 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 produce a higher boiling point (more significant in small molecules). Small molecules are soluble due to polarity (effect decreases with long chains).

Esters: no Hydrogen bonding -> very volatile, low boiling point. 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 have higher boiling points than alkanes. Tertiary amines are very similar to alkanals (but branched -> less dense packing etc).

Halogenoalkanes: Short molecules will be soluble due to polar bonds, boiling point will be somewhat higher.

[edit] 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).

[edit] 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.

RCH=CHR + H₂ -> RCH₂-CH₂R (hydrogen adds to the double bond).

RCH=CHR + Br₂ -> RCHBr-CHBrR (bromine adds onto the double bond)

RCH=CHR + HBr -> RCH₂-CHBrR (HBr adds across the double bond)

RCH=CHR + H₂O—With H3PO4 + water + 300c + 70 atm—> RC(OH)H-CH₂R.

[edit] 11.3.6

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

[edit] Initiation

2R2 -> 2R° (R° is a free radical with a lone electron)

R° + CH₂=CH₂ -> R-CH₂-CH₂°

[edit] Propagation

R~~~° + CH₂=CH₂ -> R~~~°.

[edit] Termination

R~~~° + °~~~R -> R~~~~~~R.

Polythene:
Monomer is CH₂=CH₂.
General polymer is −[−CH₂-CH₂−]_n−

Polyvinal chloride:
Monomer is CClH=CH₂ (chloroethene).
General polymer is −[−CH₂-CHCl−]_n−

[edit] 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)

CH₃CH₂OH + HOOCCH₃—H₂SO₄ and warming—> CH₃CH₂OOCCH₃ + H₂O

Esters are commonly used as artificial flavoring agents. (Mmmmmm … Ester …)

[edit] 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.

CH₃CH₂OH —oxidised by K₂Cr₂O₇/H⁺—>
CH₃CHO—oxidised by K₂Cr₂O₇/H⁺—>
CH₃COOH

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

[edit] 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).

CH₃-CO-OH + C₂H₅NH₂ -> CH₃-CO-NH-C₂H₅ + H₂O

[edit] 11.3.10

Condensation polymers

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

H₂N−(CH₂)₆−NH₂ + HOOC(CH₂)₄COOH ->
NH₂(CH₂)₆−NH−CO−(CH₂)₄−CO−NH−(CH₂)₆−NH−CO−(CH₂)₄-CO…−NH−(CH₂)₆NH₂.

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

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

HOOC-Benzene-COOH + HOCH₂CH₂OH ->
HOOC-Benzene-COOCH₂CH₂OOC-Benzene-...COOH

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.

[edit] 11.3.11

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

NH₃ + CH₃CH₂Br -> CH₃CH₂NH₂ + HBr
CH₃CH₂NH₂ + CH₃CH₂Br -> (CH₃CH₂)₂NH + HBr
(CH₃CH₂)₂NH + CH₃CH₂Br -> (CH₃CH₂)₃N + HBr
(CH₃CH₂)₃N + CH₃CH₂Br -> (CH₃CH₂)₄N⁺ + Br^-

[edit] 11.3.12

Formation of peptides and proteins to form 2-amino acids

Amino Acids: H₂N-CHR-CO-H

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

H₂N-CHX-CO-H + H₂N-CHY-CO-H + H₂N-CHZ-CO-H—>
−NH-CHX-CO-NH-CHY-CO-NH-CHZ-CO− (poly peptide)

This group can join to other peptides to form a protein.

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