Organic Chemistry
Category : UPSC
ORGANIC CHEMISTRY
Organic chemistry is the study of carbon containing compounds and their properties. This includes the great majority of chemical compounds on the planet, but some substances such as carbonates and oxides of carbon are considered to be inorganic substances even though they contain carbon. There exists a large number of organic compounds.
Differences between Organic and Inorganic Compounds
Following table compares the properties of the organic and in organic compounds:
Organic Compounds |
Inorganic Compounds |
Use mostly covalent bonding |
Mostly ionic bonding |
Are gases, liquids or solids with low melting points |
Are generally solids with high melting points |
Mostly insoluble in water |
Many are water soluble |
Many are soluble in organic solvents such as petroleum, benzene and hexane |
Most are not soluble in organic solvents |
Solution in water generally do not conduct electricity |
When dissolved in water con- ducts electrical current |
Almost all bum |
Most not combustible |
Slow to react with other chemicals |
Often undergo fast chemical reactions |
CATENATION IN CARBON
Majority of organic compounds contain chains or rings of carbon atoms that contain other elements such as O, N, P, S, Cl, Br and I. The compounds of carbon are far more numerous than the known compounds of all the other elements put together. This is because that carbon has the power to combine with other carbon atoms to form long chains; this property is not shown to such an extent by any other element. This property of carbon is known as catenation.
FUNCTIONAL GROUPS
Classes of organic compounds can be distinguished according to functional groups they contain. A functional group is a group of atoms that is largely responsible/or the chemical behaviour of the parent molecule.
Important Functional Groups and the Corresponding Classes of Organic Compounds:
S. No. |
Functional Group |
Class of compounds |
|
|
Formula |
Name |
|
1. |
\[-X\left( -F,\text{ }Cl,\text{ }-Br,\text{ }-I \right)\] |
Halo (fluoro, chloro, bromo, iodo) |
Alkyl halides or halogen compounds |
2. |
\[-OH\] |
Hydroxy |
Alcohol |
3. |
\[-OR\] |
Alkoxy |
Ethers |
4. |
\[-SH\] |
Mercapto |
Thioalcohols, mercaptans or thiols |
5. |
\[-SR\] |
|
Thioethers or sulphides |
6. |
\[-CHO\] |
Aldehydic |
Aldehydes |
7. |
\[-CO-\] |
Ketonic |
Ketones |
8. |
\[-COOH\] |
Carboxyl |
Carboxylic acids |
9. |
\[-COOR\] |
Ester |
Esters |
10. |
\[-COX\text{ }\left( X\text{ }=\text{ }Cl,\text{ }Br\text{ }or\text{ }I \right)\] |
Acyl halide |
Acid halides or Acyl halides |
11. |
\[-CON{{H}_{2}}\] |
Amide |
Amides or acid amides |
12. |
\[-CO.O.CO-\] |
Anhydride |
Acid anhydrides |
13. |
\[-N{{H}_{2}}\] |
Amino |
Amines |
14. |
\[-NH-\] |
Imino |
Imines |
15. |
\[-C\equiv N\] |
Cyano |
Cyanides or Nitriles |
16. |
\[-N\vec{=}C\] |
Isocyano |
Isocyanides or Isonitriles |
17. |
\[-N{{O}_{2}}\] |
Nitro |
Nitro compounds |
18. |
\[-N=O\] |
Nitroso |
Nitroso compounds |
19 |
\[-N=N-\] |
Azo |
Azo compounds |
20. |
\[-S{{O}_{2}}-OH\] |
Sulphonic acid |
Sulphonic acids |
HYDROCARBONS
The simplest organic compounds are the hydrocarbons, which are composed solely of carbon and hydrogen.
Hydrocarbons can be classified in two general groups aliphatic and aromatic. These compounds are the starting point for all organic compounds.
Aliphatic Hydrocarbons
Aliphatic hydrocarbons consist of straight or branched chains of carbon atoms with the other valence electrons involved in bonds with hydrogen. Examples are:
\[C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{3}}\] \[C{{H}_{2}}=C{{H}_{2}}\]
Pentane Ethene
Aliphatic hydrocarbons can be subdivided into two groups based on the types of carbon-carbon bonds the compounds contain.
Saturated aliphatic hydrocarbons
Saturated aliphatic hydrocarbons are hydrocarbons in
Which all of the carbon-carbon bonds are single bonds. These compounds are also referred to as alkanes, as mentioned for single bonds earlier. The simplest alkane is methane, \[C{{H}_{4}}\].
Unsaturated aliphatic hydrocarbons
Unsaturated hydrocarbons are those hydrocarbons, which contain at least one double or triple bond (that is, they are alkenes or alkynes) between two carbon atoms. Simplest of alkene (General formula of alkenes are:\[{{C}_{n}}{{H}_{2n-2}}\]) is ethene which consists of two double-bonded carbon atoms and four hydrogen atoms.
\[C{{H}_{2}}=C{{H}_{2}}\]
Aromatic Hydrocarbons:
Representations of Benzene
There are two accepted ways of representing the structure of benzene. The first is the Kekule structure that is written as shown below. Two structures-called resonating structures- are required, as one structure cannot fully represent the molecule. The molecule is said to alternate very rapidly between the two structures.
Properties of benzene
Benzene is totally insoluble in water, it is a volatile liquid at room temperature. The properties of other aromatics are reflective of benzene but vary according to the substituent added to the ring in place of one of the hydrogen atoms.
Handy Facts
The term “aromatic” has its origin in the fact that certain aromatic substances (for example: oil of bitter almonds, vanilla, and oil of wintergreen) contain the benzene ring. The possession of an odor is not characteristic, however, of all aromatic substances. Aromatic compounds containing benzene ring are known as benzenoids and those not containing benzene ring are known as non-benzenoids.
Aromatic hydrocarbons are the starting point for many medicinally important compounds.
ALCOHOL (ALKANOL)
Handy Facts
Methanol is very poisonous: consumption of less than two teaspoons can cause blindness. Of all the alcohols, methanol is produced in the greatest quantities in industry. Many people die due to consumption of spurious alcohol contaminated with methanol.
Properties of Alcohols
The low-molecular-weight alcohols are volatile liquids, and the high-molecular-weight alcohols (more than 13 carbons) are solids. The first three alcohols (\[{{C}_{1}}\]to\[{{C}_{3}}\]) are completely miscible (mix in any proportion) with water.
Uses of Alcohols
ETHERS
Ethers are a class of organic compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups—of general formula: R-O-R’. When the two alkyl groups are similar, then its Symmetrical ether otherwise we call it Unsymmetrical ether.
Properties of Ethers
Ether molecules cannot form hydrogen bonds with each other since they do not have a hydrogen atom attached directly to an oxygen atom. Therefore, they have about the same boiling points and melting points as alkanes of similar molecular weights.
Uses of Ethers
Ethers are used as general anesthetics in medicine. They also find use as solvents. Some important ethers include diethyl ether (which has been traditionally known as ether) that was used as a general anesthetic, and methyl phenyl ether (commonly called anisole) which is used extensively in the perfume industry.
AMINES
Properties of Amines
The low-molecular-weight amines are all volatile liquids, and those having up to five carbons are soluble in water. The element nitrogen is in the same period of the periodic table as oxygen and has some similar properties-the most significant being the ability to form hydrogen bonds. The formation of hydrogen bonds between amines, and between amines and water, accounts for their higher boiling points (than alkanes) and their water solubility.
Science in Action
Amines react with inorganic acids to from salts. (Amines react with organic acids to form amides, a class or organic compounds). This reaction results in a hydrochloride salt of the amine and is a very important reaction in pharmacy. Many drugs contain an amine functional group, and if they contain many carbon atoms, they are not very soluble in water. The salts formed from amines, however, are very soluble in water. Therefore, if we wish to use a water solution of an amine drug that is insoluble, we can make it soluble by forming the salt of the amine.
Use of Amines
Amines are very important biological compounds and are responsible for most of the fishy odours that we detect in nature. They are also found in decomposing tissues (such as off meat). Amines are building blocks of many natural and synthetic materials - proteins, textiles, plastics, adhesives, and pharmaceuticals.
ALKANOIC ACID (CARBOXYLIC ACIDS)
Carboxylic acids are compounds which contain a -COOH group.
For example ethanoic acid (also referred to as acetic acid) is commonly found in vinegar and wine. Its structural formula is\[C{{H}_{3}}COOH\].Butanoic acid) is found in cheeses, rancid butter, and under armpits of human; whilst hexanoic acid (\[{{C}_{6}}{{H}_{13}}COOH\]) is the odour associated with goats and goat cheeses. Ethanoic acid is an important industrial chemical.
Properties of Carboxylic Acids
Carboxylic acids are very polar compounds due to the two oxygen atoms and can form two hydrogen bonds between themselves.
A carboxylic acid has a higher melting point than a different type of organic compound with a similar molecular weight. Consequently, they are all solids under normal conditions.
Uses of Carboxylic Acids: Many carboxylic acids have use in food industry.
ALDEHYDE (ALKANOLS)
Aldehydes and Ketones are characterised by the presence of carbonyl group >C = O in their molecules. Aldehydes contain
Use
Handy Facts
Aldehydes are also produced in the atmosphere from hydrocarbons as a result of a complicated process called photo-oxidation. These aldehydes contribute to smog; ethanol is the aldehyde most commonly identified in smog. Methanol is almost present in all air samples tested. Aldehydes produced by plants such as 2-hexenal has been found in forested atmospheres, in addition to methanol and ethanol.
KETONES (ALKANONE)
R-C-R’
Science in Action
Ketones are also often responsible for the smell component of many natural body substances. E.g. the strange smelling breath of a diabetic is due to the presence of propanone.
ESTERS
Esters are formed from the reaction of a carboxylic acid with an alcohol and have the general structural formula RCOOR’ where R and R’ can be the same or different hydrocarbon groups.
Uses of Esters
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