organic

Carbon Chemistry


Click here for the achievement standard and links to past papers. Mindmap here.
This video is a 'rap' on organic chemistry - could be a fun way to revise.
This page covers alkanes to alcohols. Carboxylic acids, esters, fats and soaps are on the next page.

Introduction to Carbon Chemistry
carbon_electron_structure.png
Carbon electron arrangement

Carbon is element number 6. It has 6 electrons altogether, with 4 in its valence shell. The valence shell electrons are the only ones that combine with other atoms in a chemical reaction. Carbon always shares its electrons when it combines with other atoms, so it needs to share 4 electrons. When it shares electrons this is called a covalent bond. Carbon always forms four of these bonds.
Hydrogen forms one covalent bond. When 1 carbon atom reacts with hydrogen, it reacts with 4 hydrogens to make CH4. This compound is called methane (it is the major constituent of natural gas).

Naming carbon compounds

Carbon compounds follow certain naming rules so that you can tell from the name things about the compound such as how many carbons it contains and what type of compound it is. This means that when you get a name such as butanoic acid, you immedieately know certain things about it.
carbon_number.pngHow many carbon atoms: The prefix (first part) of the name tells you how many carbons are in the molecule. You will need to learn the following prefixes for the number of carbons (and know them off by heart):

Name endings: this tells you which group of organic compounds it belongs to (only the ones we study are here). You need to memorize these also

- ane for alkanes
- ene for alkenes
- ol for alcohols
- oic acid for organic acids (carboxylic acids

Examples:
Octane is the alkane with eight carbons
Ethene is the alkene with two carbons
Butanoic acid is a carboxylic acid with four carbons
Propanol is an alcohol with three carbons
Methyl pentanoate is an ester, formed between methanol (alcohol with one carbon) and pentanoic acid (carboxylic acid with five carbons)
In the next section, we will deal with the alkanes, alkenes and alcohols.

The next section will tell you about these carbon compound families and their characteristics.

Carbon compound families

1. Alkanes

These belong to the family called hydrocarbons, which contain only hydrogen and carbon. Alkanes consist of carbons joined by single bonds with hydrogen atoms on all the extra bonds.
All alkanes have the general formula CnH2n+2, where “n” is the number of carbons it has e.g
for an alkane with three carbons n is 3 the formula is C=3H=[(2x3)+2]which gives C3H8.
There are a number of different ways of representing alkane molecules. For example, this is methane, CH4
methane.png

For greater numbers of carbon atoms, the representations can get more complex and there are various ways of simplifying them:

Trends in the properties of alkanes

stategraph.png

The melting point and boiling point of alkanes increases with the number of carbons. Methane, ethane, propane and butane gases at room temperature. Butane boils at just -1ºC; propane boils at -70 ºC
  • Pentane is a liquid which boils at just 60 ºC. It is sometimes used in applications where a liquid with a low boiling point is needed - for example, it is used in the Ngawha Geothermal Power station to get more heat out of the steam; after the steam condenses, it is piped through pentane which vaporizes and drives more turbines to generate more electricity.
  • Hexane, heptane, octane and nonane are liquids and are found in petrol.
  • Heavier alkanes – C10 to C14 make up kerosene. Heavier alkanes still make up diesel and heavy fuel oils. Alkanes heavier than about C14 are solids and known as paraffin waxes.
Why does this happen: The difference in melting and boiling point within the alkane family arises for two reasons:
1. The larger the molecule, the larger its surface area; as inter-molecular forces (attractive forces between the particles) depend on surface area, this increases the melting and boiling point.
2. The increase in the mass of the molecules. It requires a certain amount of movement for the molecules to break free of the forces between them to change state and melt or evaporate. At a given temperature, the amount of energy the particles have is (on average) the same. Since kinetic energy depends on mass and speed, this means that the heavier particles must be moving slower. Therefore, it takes a higher temperature to make heavier particles move fast enough to break the forces and change state. If you were to make a molecule of butane using entirely heavy isotopes of carbon and hydrogen, it would have a higher boiling point than normal butane (isotope ratios are one of the ways geologists work out past temperatures).

Where do alkanes come from?

offshore oil/gas platform
offshore oil/gas platform
Crude oil
Crude oil
Natural gas and crude oil are mixtures of mostly alkanes. The gas component of wells of both these consists of gases methane through to butane (with carbon dioxide and other gases sometimes as impurities which need to be removed). Many gas fields produce significant quantities of light oil (generally pentane to octane isomers) termed condensate.
Gas separation: for natural gas, the butane and propane are further separated and sold as LPG because of the higher commercial value of these gases (they are valuable because they are easy to liquify and transport in tanks rather than by pipeline). The methane and ethane are sold as natural gas. These gases can be cooled and liquified as LNG, but this requires expensive infrastructure.
Methane also occurs in significant amounts in coal mines, where it can be an explosive hazard. Some coal seams (e.g at Huntly) are being exploited for their methane.

Schematic fractional distillation
Schematic fractional distillation
Methane
is also produced by anaerobic respiration in certain bacteria and is a significant component of bowel gas, and bubbles from swamps and similar environments. Such biogenic methane in the deep ocean can be trapped in the form of methane hydrates in very cold water at high pressure. Swamp gas is mostly methane; climate scientists are worried that global warming will cause increased methane production from frozen swamps called tundra, and the lakes in this terrain (Canada, Alaska and Siberia have many of these). Methane is a significant greenhouse gas, and if this happens it could cause a positive feedback pattern with further warming releasing more methane causing more warming and so on.
Petroleum and oil refining: Crude oil (or petroleum) is a mixture of alkanes from about 6 to over 30 carbons. They are separated by fractional distillation into different length alkanes for different uses. This makes use of a fractionation column as shown on the right; because alkanes with different numbers of carbons change state (liquid-gas) at different temperatures, the mixture can be separated this way. The mix of different hydrocarbons produced this way is not always what the market requires, so the additional processes of cracking and reforming are sometimes carried out. Cracking involves breaking long chain hydrocarbons down into smaller chain ones, by use of heat and catalysts. Reforming involves changing the molecular makeup of the hydrocarbons

Reactions of alkanes - combustion

There are two types of combustion – complete or incomplete. Complete combustion: produces carbon dioxide and water only and requires sufficient oxygen You need to be able to write word and formula equations for complete combustion.
carbon_equations.png
You will need to learn to balance equations like the second one. If there is a need for an online tutorial on this, please leave a message on the discussion page..
Learning object on TKI site on balancing equations - you will need your school's login and password

Incomplete combustion

Soot indicates incomplete combustion
Soot indicates incomplete combustion
Incomplete combustion occurs when there is not enough oxygen .
Incomplete combustion produces a mixture which will contain water and some or all of the following:
- carbon (soot),
- carbon monoxide (a poisonous, odourless gas),
- unburnt hydrocarbons
You can’t write an equation easily unless you are told which products are present, but if you are told one of these things is present you know combustion is incomplete. Some carbon dioxide may also be produced during incomplete combustion. If you are given one of these equations, you will have to work out how to balance it on the basis of what you are told. In practice, the products of incomplete combustion are a mixture and so the amounts don't matter.
Carbon monoxide is highly poisonous. It prevents the hemoglobin in the blood from transporting oxygen by sticking to it much more strongly than oxygen or carbon dioxide. For this reason, it is important not to use appliances that burn hydrocarbons without adequate ventilation. However, carbon monoxide also has numerous industrial applications and is manufactured for these using a variety of methods (often from coal but sometimes from methane).
Appliances such as LPG heaters, portable barbeques or any others that rely on combustion must be used with adequate ventilation, both to ensure adequate oxygen supply for combustion and to allow any CO produced to safely dispersd. Fatalities from the failure to observe this precaution are not uncommon.

Uses of alkanes

Light weight alkanes are used directly as fuels.
Methane, ethane: - natural gas
Propane - LPG
Butane - LPG (higher bp, suited for lighters and lightweight canisters), propellent for spray cans
Pentane - working fluid in secondary thermal systems (has a boiling point less than water)
Hexane to nonane - petrol
12-14 carbons - kerosene (jet fuel)
15 carbons upwards - diesel and fuel oils
20 carbons upwards - heavy fuel oils, paraffin waxes and bitumen; also cracked to make ethene and reformed into lighter alkanes for use as fuels.
A mixture of some of these heavy and light compounds forms grease and petroleum jelly, and mixed with clay and soap to make plasticene.
Author note: I have found some contradictory information on the numbers of carbons in heavier alkanes as related to their use. If anyone with suitable expertise can correct or confirm what I have written here, please contact me.

2. Alkenesalkene.jpg

Alkenes are different from alkanes in that they contain a double bond between two carbon atoms instead of a single bond. This means that two pairs of electrons are shared instead of one.
Their name always ends in –ene. The first part of the name tells you how many carbons.
Alkenes have two less hydrogens than the alkane with the same number of carbons so their general formula is CnH2n.
Because you can add more hydrogen to an alkene, it is referred to as unsaturated.

Reaction of alkenes

The double bond in ethene and other alkenes is easily broken. There are numerous reactions, but three are very important to us in this course:
1. Reaction with hydrogen to form ethane:
ethenehydrogenation.png
Because this involves the addition of something to replace the double bond, it is called an addition reaction. Halogens (chlorine, bromine and iodine) also easily react this way by binding onto the carbon.
Ethene is not usually hydrogenated like this, but unsaturated fats (fats containing double bonds) are often hydrogenated in order to increase the melting point. This is done in the manufacture of margarine and other hydrogenated vegetable oils for food manufacture.
Another addition reaction is halogenation, where ethene reacts with chloring, bromine or iodine. The iodine reaction is shown below:

halogenation.png
Since iodine is brown or purple (depending on the solvent), this results in decolourisation of the iodine solution. A similar reaction with the double bonds in unsaturate fats is used to determine the iodine number (see section on fats).
The ability to rapidly decolourise bromine water is a quick laboratory test to distinguish alkenes from alkanes.

2. Reaction with water to form ethanol:
ethanolsynthesis.png
Note: the reaction is done at high temperature and pressure; the water is in the form of steam. This is the way that most industrial ethanol is produced.
This reaction is REVERSIBLE. If ethanol vapour is passed over a strong dehydrating agent such as concentrated sulfuric acid, water will be removed and ethene will be produce. The sulfuric acid can be recycled by removing the water..

3. Polymerisation
This is when lots of individual ethene molecules join together in a long chain:
polythene.png
The result can be hundreds or thousands of units long. The individual CH2 units are termed monomers and the chain of them joined together is a polymer. The polymer made here is called polyethylene, or polythene. It is used for milk bottles, food wrap, supermarket bags and so on. A similar reaction can be undertaken with propene and the resultant molecule has a methyl group (CH3) replacing every other H on one side of the chain above. If you replace one of the hydrogens in ethene with chlorine, you have vinyl chloride. If you polymerise this the same way as ethene, you get PVC. Every other hydrogen on one side of the chain is replaced by a chlorine, giving the molecule more rigidity and stability.

Production of ethene
Ethene can be made in the lab by heating paraffin oil (e.g. baby oil) in a heatproof test tube with glass wool. This is a cracking reaction. It is performed on a larger scale with heavy oils to produce ethene for industrial synthesis reactions. It is similar in principal to the reverse of the polymerisation reaction illustrated above, but the long chain is heavy fuel oils and the chains are 'messy' (branched and so on).

Uses of alkenes

Alkenes are used as feedstock for industrial preparation of numerous chemicals. Reactions two and three above are carried out extensively on industrial scales. Propene is polymerized to make polypropylene.


Now we look at some of the non-hydrocarbon organic compounds

Alcohols

Ethanol (click for source)
Ethanol (click for source)

When the term 'alcohol' is used in general society, it refers to the chemical ethanol. To a chemist, the term alcohol refers to a homologous series of organic compounds in which one of the hydrogen atoms attached to an alkane is replaced by an -OH group (known as a hydroxyl or hydroxy group). The -OH group is termed the functional group, and alcohols can be given the general formula R-OH where 'R' stands for the 'rest' of the molecule. For example, in methanol the 'R' is a methane molecule.
There can be more than on -OH group in an alcohol; these are known as diols and triols and so on. In this Achievement Standard there is only one of these alcohols you specifically need to know about: glycerol, which is a triol. Its formal name is 1,2,3 propantriol (the name indicates that it has three -OH groups attached, and the 1,2,3 part indicates that that they occur one on each of the three carbons in the molecule). Glycerol is a by-product of soap manufacture, and we will discuss it further when we look at fats and soaps.

Physical properties of alcohols

Alcohols differ from their alkane equivalents by having a vastly higher boiling point. The boiling point of methane is around -170 degrees celsius. The boiling point of methanol is 68 degrees. This huge difference is due to the presence of forces between the molecules (inter-molecular forces). These forces attract the molecules to each other, and therefore it takes much more heat energy to pull them apart to change them into a gas.
The reason for the intermolecular forces is due to a property of the alcohol molecule called polarity. The oxygen atom attracts electrons much more strongly than carbon or hydrogen atoms (i.e. it is more electronegative). This means that the electrons in the covalent bond at the end of the molecule 'spend more time' close to the oxygen than to the adjacent hydrogen. The hydrogen atom is left a bit 'bald' - its positive charge is slightly exposed. The oxygen atom next to it is a bit more negative than it otherwise would be.
This unequal distribution of charge creates a net electric field around the -OH end of the molecule (another way of saying this is that it creates a dipole). The slightly positive hydrogen from another methanol molecule can be attracted to the slightly negative charge on the oxygen atom and vice versa. This small attractive force (a dipole-dipole force) is known as a **hydrogen bond**; it is a specific example of a more general type of intermolecular forces known as Van der Waals forces.
The hydrogen bonds are weaker than covalent bonds, but are sufficient to raise the boiling point and affect other physical properties such as specific heat capacity. They are also responsible for the fact that methanol and ethanol are completely soluble in water (and vice versa). Water is also a highly polar molecule.
The larger the alcohol molecule, the less effect the polarity of the -OH group has on the overall molecule. This means that the boiling point does not rise as steeply as that of the alkanes, because the influence of the hydrogen bonding gets progressively weaker. The miscibility (solubility) with water decreases; propanol is completely miscible with water, butanol only partly and heavier alcohols progressively less soluble in water. As their solubility in water decreases, their miscibility with non-polar solvents such as liquid alkanes increases.

Partial oxidation of alcohols
As well as complete combustion (complete oxidation by burning), alcohols can be partly oxidised into carboxylic acids by strong oxidising agents such as potassium dichromate:

acidified potassium dichromate + ethanol → ethanoic acid + chromium III ions
orange colour → green colour


This is the 'breathalyzer' reaction that was used in the days before electronic breath testing. You would not be expected to be able to write a balanced formula equation for this but you are expected to know about the colour change and the fact that the product is a carboxylic acid.
There was a question about this in the 2008 paper.

Uses of alcohols

  • Fuels: methanol and ethanol are extensively used as fuels. Both can be added to petrol; biologically derived ethanol qualifies as biofuel and is added to petrol both to improve the octane rating and to gain carbon credits. However, this practice is controversial because alcohol derived from food crops probably uses more carbon than it saves, and there are questions about its sustainability. Heavier alcohols are not widely used as fuels, though alcohols up to butanol could conceivably be added to petrol and some very heavy alcohols could be added to diesel. There are more suitable alternatives for this. You might be expected to write a balanced equation for the complete combustion of alcohols as you would for alkanes.
  • Solvents: the fact that light alcohols are miscible to a degree with water and can dissolve some non-polar substances makes them widely used as solvents e.g. for inks, perfumes and cleaning. In the first two applications, ethanol is widely used because it evaporates readily and is fairly non-toxic.
  • Germicide: The 'swine flu' threat lead to a huge increase in the sale of ethanol-based hand gel. This is 80% ethanol with a small amount of a polymer gelling agent added. It is fatal to most microbes without being unduly toxic, and the rapid evaporation of the alcohol eliminates the need for hand drying. In ancient times, wine was mixed with water as a germicide to reduce cholera and other water-borne diseases. Many mouthwashes contain ethanol, although the efficacy is dubious.
  • Chemical feedstock: for the manufacture of many organic compounds, such as esters (see next section). Bio-ethanol could conceivably be used as feedstock to make ethene for bioplastics.
  • Beverages: ethanol is found in alcoholic beverages. All primary alcohols are poisonous to varying degrees (methanol is extremely poisonous). The intoxication experienced when drinking alcoholic beverages is not harmful in small amounts but excessive or repeated use causes metabolic damage.
  • Other: Certain complex alcohols (e.g. mannitol) are used as flavours (mannitol is one of the sweeteners in sugar free gum) but you are unlikely to be asked about this. Propanol is sometimes used as 'rubbing' alcohol, often with methyl salycilate dissolved in it (linament).

Because of the amount of material, the next section is on another page.
Continued on next page with carboxylic acids, esters, fats and soaps..