ORGANIC CHEMISTRY

TIERS 5 & 6

TIER 5•Predict and explain trends in boiling points of members of a homologous series•Discuss the volatility and solubility in water of compounds containing the functional groups: compounds up to six carbon atoms with one of the following functional groups: alcohols, ketones, aldehydes, carboxylic acid, and halides .•Describe using equations the complete and incomplete combustion of alkanes•Describe using equations the reactions of methane and ethane with chlorine and bromine•Describe using equations the reaction of alkenes with hydrogen and halogens•Describe using equations, the reactions of symmetrical alkenes with hydrogen halides and water•Distinguish between alkanes and alkenes using bromine water•Describe using equations the complete combustion of alcohol•Describe using equations the oxidation of alcohols•Describe using equations the oxidation of primary and secondary alcohols•Describe using equations the substitution reactions of halogenoalkanes with sodium hydroxide

TIER 6•Explain the reactions of methane and ethane with chlorine and bromine in terms of a free radical mechanism•Outline the polymerization of alkenes•Outline the economic importance of the reaction of alkenes•Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN1 and SN2 mechanisms•Deduce reaction pathways given the starting materials and the product.

TRENDS IN PHYSICAL PROPERTIES

•Predict and explain trends in boiling points of members of a homologous series

•Discuss the volatility and solubility in water of compounds containing the functional groups: compounds up to six carbon atoms with one of the following functional groups: alcohols, ketones, aldehydes, carboxylic acid, and halides .

BOILING POINT TRENDSAs the hydrocarbon chain gets larger, the increase in number of electrons increases the temporary dipoles causing stronger van der Waals’ forces. Two features that influence the boiling point of alkanes are:

•the number of electrons surrounding the molecule, which increases with the alkane's molecular weight

•the surface area of the molecule

As a rule of thumb, the boiling point rises 20–30 °C for each carbon added

A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact with a straight chain, thus the greater van der Waals forces, between adjacent molecules.

INFLUENCE OF FUNCTIONAL GROUPS ON BOILING POINT TRENDS

Functional groups which are polar will develop dipole-dipole interactions and thus will have higher boiling points

Functional groups which enable hydrogen bonding between molecules will have even higher boiling points

The effect of functional groups on boiling points are as follows:

Most volatile (How easily to change to a gas) Least volatile

Alkane > Halogenalkane > Aldehyde > Ketone > Alcohol > Carboxylic acid

Van der Waals dipole-dipole hydrogen bonding

Increasing strength of intermolecular forces

Increasing boiling point

BOILING POINT The boiling points of other types of organic compounds are shown on the graph to the right

IMPORTANCE OF BOILING POINT TRENDS

For example, fractional distillation is used in oil refineries to separate crude oil into useful substances (or fractions) having different hydrocarbons of different boiling points. The crude oil fractions with higher boiling points:have more carbon atomshave higher molecular weightsare more branched chain alkanesare darker in colorare more viscousare more difficult to ignite and to burn

MELTING POINT TRENDSThe melting point of the alkanes follow a similar trend to boiling point and for the same reason That is, (all other things being equal) the larger the molecule the higher the melting point.

However, due to the rigidity of solids, they require more energy to break the intermolecular forces holding the molecules together The more ordered the molecule the more energy require to over come the intermolecular forces.

Odd-numbered alkanes have a lower trend in melting points than even-numbered alkanes because even-numbered alkanes pack well in the solid phase, forming a well-organized structure, which requires more energy to break apart. The odd-number alkanes pack less well and so the "looser" organized solid packing structure requires less energy to break apart.

The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane to form more organized structures.

Alkane Formula Boiling point [°C]

Melting point [°C]

Density [g·cm−3] (at 20 °C)

Methane CH4 -162 -182 gas

Ethane C2H6 -89 -183 gas

Propane C3H8 -42 -188 gas

Butane C4H10 0 -138 gas

Pentane C5H12 36 -130 0.626 (liquid)

Hexane C6H14 69 -95 0.659 (liquid)

Heptane C7H16 98 -91 0.684 (liquid)

Octane C8H18 126 -57 0.703 (liquid)

Nonane C9H20 151 -54 0.718 (liquid)

Decane C10H22 174 -30 0.730 (liquid)

SOLUBILITY IN WATERThere are two factors to consider when determining the solubilty of an organic compound in water:

•The length of the hydrocarbon chain•The nature of the functional group

Solubility decreases as the length of the chain increases

Solubility of functional groups depend on their ability to form hydrogen bonds with water

Lower members of alcohols, aldehydes, ketones, and carboxylic acids are quite soluble in water

Halogenoalkanes are not soluble in water despite their polarity b/c they do not form hydrogen bonds with water

CHEMICAL REACTIVITYDue to the nature of the strong C-C and C-H non-polar bonds, alkanes will only react in the presence of a strong source of energy.

Thus alkanes have low reactivity. However, they create highly exothermic reactions due to the fact that a large amount of energy is released forming the double bonds in CO2 and the bonds in H2O.

Alkanes burned in the presence of excess oxygen produce CO2 and H2O by the following reaction:

CxHy +O2 CO2 + H2O

If oxygen supply is limited then the reaction is :

CxHy +O2 CO + H2O

If oxygen supply is extremely limited then the reaction will be:CxHy +O2 C + H2O

Describe using equations the reactions of methane and ethane with chlorine and bromine

Step 1: Initiation

UV

Step 2: Propagation

Known as homolytic fission where the halogen molecule is broken down by UV light to produce two“free radicals “

This chain reaction both use and produce “free radicals” and so allow the reaction to continue.

Step 3: Termination

These reactions remove the free radicals and cause them to pair up their electrons

OVER ALL EQUATION:

OR

CH4 + Cl2 CH3Cl + HCl

EXAMINER’S HINT: Be sure to understand that a free radical has an unpaired electron but no net charge and an ion carries a charge

Describe using equations, the reactions of symmetrical alkenes with hydrogen halides and water

&Describe using equations the reaction of alkenes with hydrogen and halogens

Distinguish between alkanes and alkenes

BROMINE WATER TEST:Since alkenes readily undergo addition reactions ,whereas alkanes will not (they will only undergo substitution reactions in UV light), brome water is used to distinguish between the two homologous series.

Bromine water is a reddish-brown liquid that will become colorless in an alkene but remain a reddish-brown liquid in an alkane.

FLAME TEST:

Alkenes burn with a much dirtier, smokier flame that alkanes due to the fact that they have a higher C-H ratio. This causes more unburned carbon to burn in alkenes.

Benzene rings burn even dirtier

Describe using equations the complete combustion of alcohol

Alcohols react with oxygen to produce carbon dioxide and water

EXAMPLE: Combustion of Ethanol

2C2H5OH + 6O2 4CO2 + 6H2O

Describe using equations the oxidation of alcohols&

Describe using equations the oxidation of primary and secondary alcohols

The oxidation of alcohols often involves acidified potassium dichromate as the oxidizing agent. The Cr 6+ is reduced to Cr 3+. In the reaction below, the oxidizing agent is represented [O]-

Describe using equations the substitution reactions of halogenoalkanes with sodium hydroxide