|
Unlimited Tutoring & Homework Help
|
|
Introduction
Organic chemistry is that branch of chemistry which deals with the study of compounds of carbon with hydrogen (hydrocarbons), and their derivatives. Presently about five million organic compounds are known.
Characteristics of Carbon Atom
Tetravalency of Carbon
Carbon has in all, six electrons, which are distributed in first two shells, i.e., K-shell has two and L-shell accommodates four electrons viz.,
Hybridization in Carbon Compounds
sp3 hybridization (Tetrahedral hybridization)
In this type of hybridization four orbitals (one 2s and three 2p) of the excited carbon atom hybridize to form four orbitals of equivalent energy and same shape. Each orbital is called sp3 hybrid orbital.
Shapes of Molecules
Methane is the simplest carbon. Here, the four half filled orbitals belonging to the valence shell of carbon atom undergoes hybridization, resulting in the formation of four sp3 hybrid orbitals which are directed towards the corners of tetrahedron. Each of the sp3 hybrid orbitals of carbon overlaps with 1s orbital of hydrogen to form carbon-hydrogen sigma bonds. The carbon atom lies at the centre of the tetrahedron while the four hydrogens occupy the four corners or vertices of the tetrahedron.
Ionization Energy and Electronegativity
The first ionization energy of carbon atom is 1085 kJ mol-1. This value is very high. However, carbon has no tendency to lose or gain electrons as the electronegativity of carbon ,being 2.5, is neither too low nor too high. As a result, it forms bonds only by mutual sharing of electrons and thus form only covalent bonds.
Catenation
The property of self-linking with atoms of the same element is termed Catenation. Carbon has a unique property of linking itself to other carbon atoms to give open chain or/and cyclic structures. Catenation is favored by atoms where atom to atom covalent bond is quite strong.
Isomerism
Compounds having the same molecular formula, but differing in physical and chemical properties are known as isomers. This phenomenon is known as isomerism. The isomers can be identified and distinguished from one another because of difference in their physical and chemical properties.
Homologous Series
A homologous series is a family of organic compounds containing a particular characteristic group and exhibiting similar properties. For example, the compounds given below belong to the alcohol family.
CH3OH methyl alcohol (methanol)
CH3CH2CH2OH propyl alcohol 1-propanol)
CH3CH2.CH2.CH2OH butyl alcohol (1-butanol)
Functional Groups
When an atom or group of atoms bonded to a carbon atom in the chain or ring of an organic compound, shown some characteristic properties of their own, they are termed as a functional group. Thus, a functional group is an atom or group of atoms which determine the chemical behavior of an organic compound. All the compounds having a particular functional group behave alike. For example all compounds containing -OH (hydroxyl) group undergo similar reactions.
General Principles of Organic Nomenclature
Organic chemistry deals with about 5 million compounds containing carbon as an essential element. Remembering the name of each individual is not possible. Therefore, a system of nomenclature has been devised to name these compounds. Two commonly used systems of nomenclature are described below.
Naming Aliphatic Compounds
There are at least three systems in use for naming alkanes. However, the IUPAC system of nomenclature is used almost universally. According to this system, the first four members of the alkane series have special names related to their history. Thereafter, the alkanes are named after the number of carbon atoms it contained in the continuous open chain. The names of all alkanes end with the suffix '-ane'.
Naming a Complex Side Chain
If a substituent or side chain on the parent chain is complex i.e., it has a branched chain then, it is named as a substituted alkyl group. The substituted alkyl group is then named as a separate carbon chain. The carbon atom of this substituted alkyl group attached to the parent chain is numbered as 1. The name of such a substituent is enclosed in parenthesis to avoid confusion with the numbering in the parent chain. For example, the sidechain in the following molecule is named as -(1,2-dimethylpropyl).
Nomenclature of Aliphatic Hydrocarbons
Compounds containing only carbon and hydrogen are called hydrocarbons. The simplest organic compounds are the compounds of carbon and hydrogen only. There are two classes of hydrocarbons,
* Aliphatic hydrocarbons
* Aromatic hydrocarbons
Branched Chain Alkyl Groups
When a hydrogen atom from any carbon other than the terminal carbon of a carbon chain is removed, branched chain alkyl groups are obtained. Thus the removal of hydrogen atom from a secondary or tertiary carbon atom in any carbon chain results in a branched chain alkyl group. The stem name of a branched alkyl group is determined by the number of carbons they contain.
Nomenclature of Some Common Alkyl Groups
Naming Alicyclic Compounds
Aliphatic cyclic compounds are called alicyclic compounds. These compounds are named as follows:
* The number of carbon atoms in the ring determines the word root.
* The primary suffix cyclo- is added before the word root.
* The primary suffix is determined by the nature of the ring (saturated or unsaturated).
Alkenes
Alkenyl Group
The groups that are derived by removing a hydrogen atom from an alkene molecule, are called alkenyl group. The IUPAC name of such a group is obtained by replacing -e of the alkene by the suffix -yl. The numbering of the group-chain starts with the carbon attached directly to the parent chain. The few alkenyl group have trivial names which are commonly used.
Alkynes
Alkynes (earlier called acetylenes), are those unsaturated hydrocarbons containing a carbon-carbon triple bond (-C
C-). A general formula CnH2n - 2, can represent alkynes. Alkynes posses four hydrogen atoms less than the alkane and two hydrogen atoms less than alkene i.e.,
Haloalkanes (alkyl halides)
Haloalkanes (or alkyl halides) are monohalogen derivatives of alkanes. They are obtained by replacing one hydrogen atom of alkane molecule by one halogen (X) atom.
Polyhaloalkanes
Polyhaloalkanes are those halogen derivatives of alkanes, which contain more than one halogen atom in their molecule. The compounds containing two halogen atoms are termed dihalo and those containing three halogen atoms as tri halo, and so on.
Alkanols (Saturated Monohydric Alcohols)
Alkanols or monohydric alcohols are further classified as primary, secondary and tertiary alcohols. The classification is according to the OH group that is attached to a carbon atom, which in turn is attached to one (or none), two or three carbon atoms respectively. For example,
Alkanediols (Dihydric Alcohols)
Dihydric alcohols called as glycols or alkanediols are the dihydroxy derivatives of alkanes. In an alkanediol (dihydric alcohols) the two -OH groups are attached to different carbon atoms. The common and IUPAC names of dihydric alcohols are obtained as follows. The common name of a glycol is derived from the name of the corresponding alkene.
Alkanetriols (Trihydric Alcohols)
Trihydric alcohols are organic compounds whose molecule contains three hydroxy (-OH) groups. The trihydroxy derivatives of alkanes are termed alkanetriols. Three -OH groups are bonded to three different carbon atoms. The most important trihydric alcohol is glycerol.
Alkoxyalkanes (Ethers)
Ethers are alkoxy derivatives of alkanes. These are represented by the general formula R-O-R', where R and R' are alkyl groups. Ether is termed as simple ether when R and R' are the same. If the two alkyl groups (R and R') are different, the ether is termed as a mixed ether.
Alkanals (Aldehydes)
The common name of the acid that an aldehyde gives on oxidation, gives the aldehyde its common name. This is done by replacing the ending -ic acid in the name of the acid by the word aldehyde.
Alkanones (Ketones)
The two alkyl groups (R and R') may be same or different. When the two alkyl groups (R and R') are same, the ketone is a simple ketone. If the two are different the ketone is called a mixed ketone.
Alkanoic Acids or Saturated Monocarboxylic Acid
On replacing one H atom of an alkane by one carboxylic (- COOH) group a saturated monocarboxylic acid or alkanoic acid is obtained. Higher members of this family are generally, known as fatty acids (obtained by the hydrolysis of fats).
Alkanamides (Acid Amides)
The common names of acid amides are obtained by replacing the suffix -ic acid of the corresponding acid by the word amide.
Alkanoyl chlorides (Acyl Chlorides or Acid Chloride)
Acid chlorides contain the functional group -COCl. This functional group is obtained by replacing the -OH group of a carboxyl group with a chlorine atom. The common name of an acid chloride is obtained by replacing the ending -ic acid from the common name of the parent acid with -yl chloride.
Alkanoic anhydrides (acid anhydrides)
An acid anhydride may be considered as the condensation product of two molecule of a monocarboxylic acid obtained by losing one molecule of water.
Alkylalkanoates (esters)
An ester contains the group COOR where R is an alkyl group. The hydrolysis of the ester gives the parent acid and so its name is based on the name of the acid.
Nitroalkanes and Alkyl Nitrites
Nitro compounds are named as the nitro derivatives of the parent alkane in both the systems of nomenclature. This is done by adding the prefix nitro to the word root of the parent alkane.
Alkanamines (Amines)
Amines are formed by replacing one or more hydrogen atoms of ammonia (NH3) with alkyl groups. These are further classified as primary, secondary and tertiary amines according to one, two or three hydrogen atoms of ammonia being replaced by alkyl groups.
Alkane nitriles (Alkyl Cyanides)
The common name of an alkyl cyanide is obtained by adding the word cyanide to the name of the alkyl group. Sometimes, an alkyl cyanide is also named as the nitrile of the acid which is produced on its hydrolysis. It is named by changing the suffix -ic acid by -onitrile.
Alkylcarbylamines (Alkyl Isocyanides)
In common system of nomenclature, these compounds are either named as alkyl isocyanide or as isonitrile of the acid produced on hydrolysis by changing the 'ic' acid by 'isonitrile'.
Naming Compounds with More Than One Similar Functional Groups
When a molecule contains more than one similar functional groups, then, the word di (for 2), tri (for 3), etc., is added before the secondary suffix, of the functional group. While adding such words, the vowel 'e' of primary suffix is retained. For example:
Naming Compounds With More Than One Different Functional Groups
When the molecule contains two or more dissimilar functional groups, the parent chain is so chosen so as to contain maximum possible number of functional groups. The numbering of the parent chain is done so that the functional group of higher priority gets the lower number.
Naming Aromatic Compounds
Aromatic compounds are those carbocyclic compounds that contain at least one benzene ring. The ring of six carbons in benzene or any of its derivatives is termed benzene ring or nucleus. A benzene ring may be represented in any of the following ways.
Types of Aromatic Compounds
Nuclear Substituted Compounds
When the functional group or any substituent, in aromatic compounds is directly attached to the benzene ring, it is a called nuclear substituted compound. Such compounds are named as the derivatives of benzene under the IUPAC system. However, the common names of many such compounds are retained by IUPAC.
Electron Displacement in Covalent Bonds
Organic compounds mainly consist of covalent bonds. The displacements of the electron pairs in these covalent bonds may take place either on their own or under the influence of other species. Three types of electron displacements are generally noticed in the mechanism of organic reactions.
Electromeric Effect
This is a temporary effect and takes place between two atoms joined by a multiple bond, i.e., a double or triple bond. It occurs at the requirements of the attacking reagent, and involves instantaneous transfer of a shared pair of electrons of the multiple bond to one of the linked atoms.
Resonance or Mesomeric Effect
If two or more structures can be assigned to a molecule, none of which is capable of describing all the known properties of the compound, then the actual structure is known to be a resonance hybrid or an intermediate of these structures. The various structures written are called resonating structures and this phenomenon is called resonance.
Important Feature of Resonance
The various contributing structures should have the same arrangements of structures but may differ only in electronic arrangements.
Hyperconjugation
When atoms / groups having lone pair of electrons are bonded by a single bond to the conjugated system, they displace electrons towards multiple bonds.
Types of Bond Fission
Homolytic Fission
When the cleavage of covalent bond between two atoms takes place in a manner, which enables each atom to retain one electron of the shared pair, it is known as homolytic fission. This fission is symmetrical and leads to the formation of atoms or groups of atoms having unpaired electrons, called free radicals. The free radicals are denoted by putting dot over the symbol of atom or group of atoms. For example,
Carbocation or Carbonium Ion
It is defined as a group of atoms, which contain positively charged carbon having only six electrons. A carbonium ion is obtained by heterolytic fission of a covalent bond involving carbon atom.
Relative Stability of Carbocation
We know that methyl group is electron releasing i.e. has +I inductive effect. The alkyl group attached to positively charged carbon atom tends to release electrons towards carbon. This decreases the positive charge on the carbon atom but the carbon itself is somewhat positive. As a result, the positive charge on the carbon atom gets dispersed, giving it stability. Therefore, more the number of alkyl groups, the greater will be the dispersal of charge and therefore, more stable will be the carbocation.
Carbanion
A species containing a carbon atom carrying a negative charge may be defined as carbanion. These are generated by the heterolytic fission of covalent bond involving carbon atom, where the atom linked to carbon goes away without the bonding electrons. This causes the carbon to acquire a negative charge.
Carbenes
Carbenes are reactive neutral species in which the carbon atom has six electrons in the valence shell, two of which are shared. The simplest carbene is methylene (CH2). It is formed when diazomethane is decomposed by the action of light. It is very reactive.
Types of Attacking Reagents
Positively charged or neutral species, which are deficient of electrons and can accept a pair of electrons are called electrophiles. These are also called electron loving (philic) species.
Types of Organic Reactions
The reactions in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or group of atoms are called substitution reaction.
