Chemistry Chronology: Topic 11

11.1 Alkanes
11.2 Alkenes
11.3 Alcohols
11.4 Carboxylic acids
11.5 Polymers
*The use of molecular models is recommended to enable students to appreciate the three-dimensional structures of molecules.
Hydrocarbons are compounds that are made up of atoms of carbon and hydrogen as the only elements.
(a) state that the naphtha fraction from petroleum (crude oil) is the main source of hydrocarbons used as the feedstock for the production of a wide range of organic compounds
Naphtha is a flammable liquid made from distilling petroleum. It looks like gasoline. Naphtha is used to dilute heavy oil to help move it through pipelines, to make high-octane gas, to make lighter fluid, and even to clean metal.

(b) describe the issues relating to the competing uses of oil as an energy source and as a chemical feedstock

Crude oil is an important source of:

fuels such as petrol, diesel, kerosene, heavy fuel oil and liquefied petroleum gases
feedstock for the petrochemical industry

Crude oil is a finite resource. Petrol and other fuels are produced from it using fractional distillation. Cracking is used to convert long alkanes into shorter, more useful hydrocarbons.

A feedstock is a raw material used to provide reactants for an industrial reaction. A petrochemical is a substance made from crude oil using chemical reactions. For example, ethene is produced from crude oil. It is used as feedstock to make poly(ethene), a polymer.

Other useful substances made from compounds found in crude oil include:

· solvents
· lubricants
· detergents

11.1 Alkanes
(a) describe a homologous series as a group of compounds with a general formula, similar chemical properties and showing a gradation in physical properties as a result of increase in the size and mass of the molecules, e.g. melting and boiling points; viscosity

Each crude oil fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. They have similar (but not identical):

· numbers of hydrogen and carbon atoms in their molecules

· boiling points

· ease of ignition

· viscosity

For example, the gases fraction contains hydrocarbons with one to four carbon atoms. These have:

· boiling points below room temperature

· they are very flammable

· have a low viscosity

The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have:

· boiling points well above room temperature
· are very difficult to ignite
· have a high viscosity

(b) describe the alkanes as a homologous series of saturated hydrocarbons with the general formula C_nH_2n + 2

The alkanes form a homologous series. Like all homologous series, the alkanes:

· have the same general formula
· differ by CH₂ in molecular formulae from neighbouring compounds
· show a gradual variation in physical properties, such as their boiling points
· have similar chemical properties

General formula

The general formula for the alkanes is C_nH_2n₊₂, where n is the number of carbon atoms in the molecule.

(d) define isomerism and identify isomers

(e) describe the properties of alkanes (exemplified by methane) as being generally unreactive except in terms of burning and substitution by chlorine

Chemical properties of Alkanes

Alkanes are comparatively, inactive compounds except under certain conditions such as burning, substitution and thermal catalytic cracking.

Burning

All alkanes burn giving water vapour, carbon dioxide and heat, these reactions are highly exothermic, that is why they are used as fuels.

CH_{4 (g)}+ 2 O_{2 (g)}→ CO_{2 (g)} + 2H₂O_(l)+ Heat

Substitution

Alkanes react with halogens (F₂, Cl₂, Br₂) by heating up to 400°C or in presence of indirect sunlight (Ultraviolet rays) in series of substitution reactions, The difference in the products of halogenation of alkanes in the presence of ultraviolet rays because the product depends on the ratio between methane and halogen in reaction mixture,

Substitution reaction is the type of reaction between methane & halogens in the presence of ultraviolet rays.

CH_{4 (g)}+ Cl_{2 (g)}→ CH₃Cl_(g) + HCl_(g)

CH₃Cl_(g) + Cl_{2 (g)}→ CH₂Cl_2 (g) + HCl_(g)

CH₂Cl_2 (g) + Cl_{2 (g)} → CHCl_3 (g) + HCl_(g)

CHCl_3 (g) + Cl_{2 (g)}→ CCl_4 (g) + HCl_(g)

In case of direct sun rays (Elimination), the reaction is accompanied by an explosion.

CH_{4 (g)}+ 2Cl_{2 (g)}→ C_(s) + 4 HCl_(g)

11.2 Alkenes
(a) describe the alkenes as a homologous series of unsaturated hydrocarbons with the general formula C_nH_2n and containing the C=C functional group

The alkenes form a homologous series. Like all homologous series, the alkenes:

· have the same general formula

· differ by CH₂ in molecular formulae from neighbouring compounds

· show a gradual variation in physical properties, such as their boiling points

· have similar chemical properties

General formula

The general formula for the alkenes is C_nH_2n, where n is the number of carbon atoms in the molecule.

(b) draw the structures of b ranched and unbranched alkenes, C2 to C4, and name the unbranched alkenes, ethene to butene

(c) describe the manufacture of alkenes and hydrogen by cracking hydrocarbons and recognise that cracking is essential to match the demand for fractions containing smaller molecules from the fractional distillation of petroleum (crude oil)

Cracking is important for two main reasons:

1. It helps to match the supply of fractions with the demand for them.

2. It produces alkenes, which are useful as feedstock for the petrochemical industry.

Supply and demand

The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Very often, fractional distillation of crude oil produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.

Smaller hydrocarbons are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.

Thermal catalytic cracking

This process usually takes place during the refining of petroleum oil to convert the heavy long petroleum chains (less used) to the daily used lighter short chain products, this process takes place by heating the heavy petroleum products under high pressure and temperature in the presence of a catalyst to produce two kinds of products.

1. Short chain alkanes which are used with gasoline to fulfill the permanent world needs such as car fuel.

2. Short chain alkenes as ethene and propene which are used in many chemical industries such as the manufacture of polymers.

C₈H_{18 (l)} → C₄H_{8 (g)}+ C₄H_{10 (g)}

Thermal catalytic cracking (for octane only) is a process in which the long carbon chains are broken into shorter ones by the action of heat, pressure and catalysts.

(d) describe the difference between saturated and unsaturated hydrocarbons in terms of their structures and in their reaction with aqueous bromine

The alkenes are unsaturated hydrocarbons:

· hydrocarbons, because they are compounds containing hydrogen and carbon only

· unsaturated, because they contain a C=C double bond, which means that they have two fewer hydrogen atoms than the corresponding alkane

The C=C bond is the functional group in the alkenes. It is responsible for the typical reactions of alkenes.

Saturated hydrocarbons have a single bonds between their atoms, which means only one pair of electrons is shared between any two atoms of the compound.
Unsaturated hydrocarbons can have multiple bonds between carbon atoms.

(e) describe the properties of alkenes in terms of combustion, polymerisation and their addition reactions with bromine, steam and hydrogen

Chemical Properties of Alkenes

Alkenes are unsaturated compounds, which makes them highly reactive. Most of these chemical reactions occur at the Carbon-Carbon double bonds. This makes alkenes far more reactive than alkanes. Alkenes undergo three types of main reactions, which are as follows

Addition Reactions

· Addition of Hydrogen: In the presence of nickel or platinum alkenes will react to add to its molecular chain one diatomic molecule of hydrogen (dihydrogen). And in this process, they become alkanes due to the rearrangement of atoms.

· Addition of Halogens: Halogens will react with alkenes to form dihalides. From the halogens, iodine will not react with alkenes. But Bromine reacts with alkenes and will attach at the unsaturated site. In fact, the reaction is used to as proof of unsaturation.

C₂H₄(g) + Br₂ (aq) → C₂H₄Br₂ (aq)

· Addition of Halides: when a hydrogen halide will react with an alkene, the hydrogen will attach at the double bond to the atom with more hydrogen atoms attached. The halide ion, on the other hand, will attach to that carbon atom that has the lesser hydrogen atoms attached.

CH₃-CH=CH₂+ HBr → CH₃-CH(Br)-CH₃

· Addition of Water: water will react with an alkene to form alcohols. This happens in the presence of sulphuric acid.

CH₂=CH₂ + H₂O → CH₃CH₂OH

Oxidation Reactions

• Combustion Reaction: The combustion of alkenes is very exothermic, it will give out huge amounts of thermal energy. A practical example of this reaction is seen in welding of metals. It is known as oxy-ethylene welding.

CH₂=CH₂ + 3O₂ → 2CO₂ + 2H₂O

• Oxidation by Pottasium Permanganate: When alkenes are reacted with cold dilute KMnO₄ they will decolourize the pink colour of KMnO₄. So it is used for testing unsaturation in compounds.

Polymerization

Polymerization is the combination of a huge number of unsaturated simple molecules (monomers), their number ranges from 100 to 1000,000 to form a large molecule (polymer) which has the same empirical formula of the original compound, Empirical formula is the formula which shows the smallest ratio of atoms inside the molecule.

Polymerization takes place for small unsaturated molecules, Each small unsaturated is called Monomer, When two monomers combine together, they form Dimer, When a dimer combine with a monomer, they form Trimer, When a huge number of monomers combine together, they form a Polymer.

Alkenes are characterized by their ability to form polymers by addition.

Addition polymerization takes place by the combination of a huge number of similar unsaturated small molecules (monomers) to each other to form a very large molecule (polymer) such as the formation of polyethylene.

(f) state the meaning of polyunsaturated when applied to food products
There are two main types of fats — saturated and unsaturated
A saturated fat has no double bonds in its chemical structure, whereas an unsaturated fat has one or more double bonds.

If a fat molecule has one double bond, it’s called a monounsaturated fat, but if it has more than one, it’s called a polyunsaturated fat.

Polyunsaturated fats — along with monounsaturated fats — are considered healthy fats, as they may reduce your risk of heart disease, especially when substituted for saturated fats The two major classes of polyunsaturated fats are omega-3 and omega-6 fatty acids. Both are essential fatty acids that the body needs for brain function and cell growth. Yet, the body cannot make essential fatty acids, so they must be obtained from the meals.
Polyunsaturated fats are usually liquid at room temperature and are referred to as “oils.” They’re found mostly in fatty fish, plant-based oils, seeds and nuts.

Most of the fat in butter is saturated, but it also contains some mono- and polyunsaturated fats. Some foods provide a higher percentage of omega-3 and omega-6 polyunsaturated fats than others. Here are several foods high in these essential fatty acids.

Omega-3s is found in pine nuts, walnuts, flax and sunflower seeds — but these give a less active form of the fat than fish do.

Fatty fish, such as salmon, boast the most omega-3s, whereas fish with a lower fat content, such as trout and bass, harbour lower levels.

The omega-3 content of 3 ounces (85 grams) of selected fish is:

Salmon: 1.8 grams

Herring: 1.7 grams

Sardines: 1.2 grams

Mackerel: 1 gram

Trout: 0.8 grams

Bass: 0.7 grams

Shrimp: 0.2 grams

Fish don’t produce omega-3 fatty acids on their own. Instead, they accumulate them by eating algae and small, microscopic organisms called plankton.

Plant-based oils are high in omega-6 fatty acids — with the exception of coconut and palm oil, which contain a high percentage of saturated fats and are solid at room temperature.

The oils highest in polyunsaturated fats include

Safflower oil: 74.6%
Grapeseed oil: 69.9%
Flaxseed oil: 67.9%
Sunflower oil: 65.7%
Poppyseed oil: 62.4%
Soybean oil: 58.9%

These oils are liquid at room temperature because the double bonds allow the fat to bend and fold. Oil-based condiments like mayonnaise and salad dressings, as well as margarines, are also high in omega-6 polyunsaturated fats.

As an essential component of the diet, polyunsaturated fats offer many impressive health benefits.

Much of these benefits are associated with the omega-3 fatty acids EPA and DHA. These may Reduce Age-Related Mental Decline.

Omega-3 fatty acids are crucial for brain development and function.

Observational studies link low blood levels of DHA with mental decline in older adults

On the other hand, eating fish — which is high in DHA — may help prevent mental decline and related illnesses.

if you wanted to replace some of your saturated fats with polyunsaturated fats, you could cook and bake with liquid oils instead of butter, lard or shortening, which are high in saturated fats.

Polyunsaturated fats spoil more quickly than other fats because of their multiple double bonds.

Therefore, you should store these oils in a cool, dark place before opening, after which you should keep them in the refrigerator.

Polyunsaturated fats also have a lower smoke point, which is the temperature at which an oil starts to smoke.

When oil smokes, its fat breaks down and produces harmful substances, some of which have been linked to cancer and neurodegenerative diseases in animal studies.

(g) describe the manufacture of margarine by the addition of hydrogen to unsaturated vegetable oils to form a solid product

Margarine is typically used as a substitute for butter, for spreading on bread and in baking. There are many different brands of margarine containing different blends of oils and fats. Avoid buying any margarine with ‘trans’ or hydrogenated fats as these are particularly bad. Research has suggested that margarines containing plant sterols and stanols can help to reduce blood cholesterol by as much as 10% and therefore these may be beneficial for some people. Such margarines may not be suitable for children though, so always read the label carefully.

11.3 Alcohols
(a) describe the alcohols as a homologous series containing the –OH functional group
(b) draw the structures of alcohols, C1 to C4, and name the unbranched alcohols, methanol to butanol
(c) describe the properties of alcohols in terms of combustion and oxidation to carboxylic acids
(d) describe the formation of ethanol by the catalysed addition of steam to ethene and by fermentation of glucose

(e) state some uses of ethanol, e.g. as a solvent; as a renewable fuel; in the production of vinegar
11.4 Carboxylic acids
(a) describe the carboxylic acids as a homologous series containing the –CO₂H – functional group
(b) draw the structures of carboxylic acids, methanoic acid to butanoic acid, and name the unbranched acids, methanoic to butanoic acids
(c) describe the carboxylic acids as weak acids, reacting with carbonates, bases and some metals

(d) describe the formation of ethanoic acid by the oxidation of ethanol by acidified potassium manganate(VII) and the formation of vinegar by bacterial oxidation
(e) describe the reaction of carboxylic acids from C1 to C4 with alcohols from C1 to C4 to form esters
(f) draw the structures of and name the esters formed from carboxylic acids (see 11.4 (b)) and alcohols (see 11.3 (b))
(g) state some commercial uses of esters, e.g. perfumes; flavourings; solvents
11.5 Polymers
(a) describe polymers as large molecules made from many small units called monomers, different polymers having different repeat units and/or different linkages

(b) describe the formation of poly(ethene) as an example of addition polymerisation of ethene as the monomer

(d) describe nylon, a polyamide, and Terylene, a polyester, as condensation polymers, the partial structure of nylon being represented as

(details of manufacture and mechanisms of these polymerisations are not required)

(e) state some typical uses of synthetic fibres such as nylon and Terylene, e.g. clothing; curtain materials; fishing line; parachutes; sleeping bags

(f) deduce the partial structure of the polymer product from a given monomer and vice versa
(g) describe the pollution problems caused by the disposal of non-biodegradable plastics