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IAQUK Resources - Basic Organic Chemistry

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Specifying carbon chain length

 

The simplest organic compounds are built of straight chains of carbon atoms which are named by means of prefixes that denote the number of carbons in the chain. Using the convention Cn to denote a straight chain of n atoms, the prefixes for chain lengths from 1 through 10 are given here:

 

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

meth-

eth-

prop-

but-

pent-

hex-

hept-

oct-

non-

dec-

 

As you can see, chains from C5 onward use Greek number prefixes, so you don't have a lot new to learn here. The simplest of these compounds are hydrocarbons having the general formula CnH2n+. They are known generically as alkanes, and their names all combine the appropriate numerical prefix with the ending -ane:

 

CH4

C2H6

C3H8

C8H18

C

C–C

C–C–C

C–C–C–C–C–C–C–C

methane

ethane

propane

octane

 

Since organic (carbon) compounds constitute the vast majority of all known chemical substances, IAQUK have provided some basic details about understanding how each distinct compound has a unique molecular structure which has a unique characteristic and name.

Sources


Organic compounds are derived from three sources:

 

  • Nature: fibers, vegetable oils, animal oils and fats, alkaloids, cellulose, starch, sugars, and so on.
  • Synthesis: A wide variety of compounds and materials prepared by manufacturing processes.
  • Fermentation: Alcohols, acetone, glycerol, antibiotics, acids, and the like are derived by the action of microorganisms upon organic matter.

Carbon (C) appears in the second row of the periodic table and has four bonding electrons. Organic chemicals get their diversity from the many different ways carbon can bond to other atoms. The simplest organic chemicals, called hydrocarbons, contain only carbon and hydrogen atoms; the simplest hydrocarbon (called methane) contains a single carbon atom bonded to four hydrogen atoms.

 

But carbon can bond to other carbon atoms in addition to hydrogen. In fact, the uniqueness of carbon comes from the fact that it can bond to itself in many different ways. Carbon atoms can form long chains, branched chains and rings.  There appears to be almost no limit to the number of different structures that carbon can form.  To add to the complexity of organic chemistry, neighbouring carbon atoms can form double and triple bonds in addition to single carbon-carbon bonds.

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Hydrocarbon

 

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon.

 

There are two types, saturated and unsaturated. Saturated hydrocarbons are those in which adjacent carbon atoms are joined by a single covalent bond and all other bonds are satisfied by hydrogen.

 

Unsaturated hydrocarbons have at least two carbon atoms that are joined by more than one covalent bond and all remaining bonds are satisfied by hydrogen.

 

Saturated Hydrocarbons


The saturated hydrocarbons form a whole series of compounds starting with one carbon atom and increasing one carbon atom, stepwise. These compounds are called alkanes, or the methane series. The principal source is petroleum.

 

The hydrocarbons serve as feedstocks for the preparation of a wide variety oforganic chemicals. This knowledge serves as the basis of the great petrochemical industry within the petroleum industry. Saturated hydrocarbons are quite inert toward most chemical reagents. For this reason they were termed “paraffins” by early chemists (from the Latin parum affinis, meaning “little affinity”).

 

Chemical Reactions

Strong bases, acids, or aqueous solutions of oxidizing agents do not react with saturated hydrocarbons at room temperature. At elevated temperatures, strong oxidizing agents, such as concentrated sulfuric acid, oxidize the compounds to carbon dioxide and water. Other reactions of importance are as follows:

 

  • Oxidation with oxygen or air.


  • Substitution of hydrogen by halogens:
  • This reaction does not ordinarily occur in aqueous solutions and therefore is of little significance in environmental engineering and science.


  • Pyrolysis or cracking:
  • High-molecular-weight hydrocarbons may be broken into smaller molecules by heat treatment. The process is used in the petroleum industry.


  • Biological oxidation:
  • Hydrocarbons are oxidized by certain bacteria under aerobic conditions.

 

 

Unsaturated Hydrocarbons

 

The unsaturated hydrocarbons are usually separated into four classes.

 

Alkenes

Each member of the alkane group except methane can lose hydrogen to form an unsaturated compound or alkene. The alkenes all contain one double bond between two adjacent carbon atoms, and their names all end in -ylene or -ene. The alkenes are also called olefins. Alkenes, particularly ethene, propene, and butene are formed in great quantities during the cracking or pyrolysis of petroleum.

 

Diolefins

When aliphatic compounds contain two double bonds in the molecule, they are called diolefins, some times dienes for short. The compound 1,3-butadiene is an important example, which has been used to make polymers.

 

Alkadienes

Some organic compounds contain more than two double bonds per molecule. The red colouring matter of tomatoes, lycopene, and the yellow colouring matter of carrots are examples.

 

Alkynes

The alkynes have a triple bond between adjacent carbon atoms. These compounds are found to some extent in industrial wastes from certain industries, particularly those from the manufacture of some types of synthetic rubber.

 

Chemical Reactions

Unsaturated linkages occur in many types of organic compounds and exhibit many properties in common, regardless of the type of compound in which they exist. Unsaturated compounds undergo several reactions with relative ease.

 

  • Oxidation: The compounds are easily oxidized in aqueous solution by oxidizing agents such as potassium permanganate. A glycol is the normal product.


  • Reduction: Under special conditions of temperature, pressure, and catalysis, hydrogen may be caused to add at double or triple bonds.


  • Addition: Halogen acids, hypochlorous acid, and halogens will add across unsaturated linkages. The reaction with hypochlorous acid is most important. Industrial wastes containing appreciable amounts of unsaturated compounds exhibit high chlorine-demand values because of such reactions.


  • Polymerization: Molecules of certain compounds having unsaturated linkages are prone to combine with each other to form polymers of higher molecular weight. Similar reactions serve as the basis for many industrial products, e.g. synthetic resins, synthetic fibre, synthetic rubber, and synthetic detergents.


  • Bacterial oxidation: It is generally considered that organic compounds possessing unsaturated linkages are more prone to bacterial oxidation than corresponding saturated compounds because of the ease of oxidation at the double bonds.

Chains and Rings

 

In organic chemistry, there are a few basic structural shapes that you will encounter. They are chains and rings.


There are also two types of chains, a straight chain, and a branched chain. In a straight chain, one carbon atom holds no more than two other carbon atoms. As its name implies, the straight chain is a straight link of carbon, sometimes oxygen or nitrogen, atoms, in structural formula that is. Because of twisting and contouring, they chain may have several conformations.


Branched chains have at least one carbon holding more than two other carbon atoms. It will, as its name implies, have branches of other chains coming off another chain. Branching is one of the reasons why there are so many isomers for each compound.

 

Rings (or cyclic compounds) are composed of rings of carbon and sometimes oxygen or nitrogen. For example, Benzen.

Isomers

 

Isomerism is another reason why there are so many organic compounds. Isomers are compounds with identical molecular composition but their structures are arranged differently. Depending on how they are arranged, they may have similar or different properties. 

 

Because the number of carbons per molecule increases as the compound gets more complex, the number of possible isomers for any given formula becomes very, very large.

 

 

Formula

Number of Isomers

C8H18

18

C10H22

75

C20H42

366,319

Benzene Derivatives

 

The nomenclature of substituted benzene ring compounds is less systematic than that of the alkanes, alkenes and alkynes. A few mono-substituted compounds are named by using a group name as a prefix to "benzene", as shown by the combined names listed below. A majority of these compounds, however, are referred to by singular names that are unique. There is no simple alternative to memorization in mastering these names.

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