## 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, $C{{H}_{4}}$.

• All other alkanes are formed by adding$C{{H}_{2}}s$ to the formula. Hence the general formula for the alkanes is ${{C}_{n}}{{H}_{2n+2}}$.
• Alkanes are not very reactive chemically and are insoluble in water. They bum to form carbon dioxide and water (combustion reaction).
• Alkanes also under go reactions induced by UV light. Examples include the slow breakdown of plastics in the sun, and halogenations reactions (reactions where halogen atoms such as Cl, Br, or I replace H in the molecule).

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.

$C{{H}_{2}}=C{{H}_{2}}$

• The general formula of alkyne is${{C}_{n}}{{H}_{2n}}$ and the simplest of alkyne is acetylene CH=CH

Aromatic Hydrocarbons:

• The second major group of the hydrocarbons is the aromatic hydrocarbons, which are hydrocarbons that contain a benzene ring as part of their structure.
• Benzene has the formula ${{C}_{6}}{{H}_{6}}$ and consists of six carbon atoms in a ring with three alternating double bonds.
• Aromatic compounds show completely different behaviour to aliphatic compounds, and hence it is not difficult to deduce that they have a special chemical structure.

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)

• Alcohols are hydroxyl (-OH) derivatives of hydrocarbons formed by replacing a hydrogen with the hydroxyl radical and are of the general form R-OH where R represents the hydrocarbon.
• There are three categories of alcohols: primary, secondary, and tertiary.

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.

• Ethanol ($C{{H}_{3}}C{{H}_{2}}OH$) has been prepared since antiquity by fermentation of sugars and starches, catalyzed by yeast. Sugars for fermentation come from a variety of sources, including grains, grape juice, various vegetables and agricultural wastes.
• Yeast secrets two enzymes called inverses and zymase. These enzymes act as catalyst for converting sugar to ethanol.

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

• Alcohols are most commonly used as solvents in the pharmacy.
• They are also used as disinfectants and antiseptics.

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

• Amines result from the replacement of one or more of the hydrogen atoms of ammonia with hydrocarbons and have the general formula$R-N{{H}_{2}}$.
• Amines are derivatives of ammonia that has three hydrogen.
• Amines are classified according to the number of hydrogen that are replaced by alkyl or phenyl groups in ammonia ($N{{H}_{3}}$)
• Whenever one of the hydrocarbon groups connected to the nitrogen atom contains a benzene ring, the compound is referred to as an aromatic amine.

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.

• Most of the simpler saturated carboxylic acids are found in nature.

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.

• Alkanoic acids (Carboxylic acids) are produced in major quantities by the combustion of coal and wood. Long chain carboxylic acids have also been found in the atmosphere, the major source of the compounds being pollens and other plant products.

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.

• Benzoic acid - a common preservative used in beverages.
• Citric acid - used to provide a sour taste in fruit and vegetables.
• Sorbic acid - used as an acidic anti-microbial agent.
• Lactic acid - commonly found in cultured dairy products.
• Malic acid - commonly found in fruits.
• Tartaric acid - used extensively in cooking processes (it is also called cream of tartar)

ALDEHYDE (ALKANOLS)

Aldehydes and Ketones are characterised by the presence of carbonyl group >C = O in their molecules. Aldehydes contain

• Aldehydes have lower boiling points than corresponding alcohols or acids as they cannot form hydrogen bonds between themselves.
• The lower-molecular-weight aldehydes (up to five carbons) are soluble in water. Aldehydes are neutral in pH and undergo both oxidation and reduction reactions.
• They are easily oxidized to acids and reduced to alcohols.

Use

• Some aldehydes, such as vanillin and Benz aldehyde, are frequently used in the pharmacy as flavouring agents. Others, such as formaldehyde, are often used as disinfectants.
• Both aldehyde (and ketones) occurs in natural and man-made products very frequently.
• All naturally occurring plant sugars (such as glucose) are aldehydes, ketones, or their products.

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)

• Ketones result from the oxidation of a secondary alcohol and have the general structural formula O where R and R’ can be the same or different hydrocarbon groups.

R-C-R’

• Ketones are similar to aldehydes in their boiling points, which are lower than those of corresponding alcohols and carboxylic acids.
• Ketones are neutral compounds, being neither acids nor bases.

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.

• The simplest esters are liquids and have fragrant odours. An example is ethyl acetate, $C{{H}_{3}}-C{{H}_{2}}-OOC-C{{H}_{3}}$, which has the odour of pineapple.
• Esters have boiling points similar to alkanes of similar molecular weight as they cannot form hydrogen bonds between themselves.
• They can form hydrogen bonds with water. Therefore, esters that contain less than five carbon atoms are soluble in water.

Uses of Esters

• The sweet and pleasant odours and tastes of many foods are due to complex mixtures of organic compounds, of which esters are generally the most prevalent component. Some examples of esters that are used as flavouring agents and the corresponding flavours are:
• methyl butanoate : apple
• ethyl butanoate : strawberry
• butyl butanoate : pineapple
• pentyl ethanoate : banana
• methyl 2-phenylethanoate: jasmine

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