Open chain and cyclic compounds

Open chain and cyclic compounds

As we know, there are number of compounds with open chain and closed rings comprised of carbon atom only. Thus, the compound made up of only carbon and hydrogen elements is known as hydrocarbons. Other organic compounds are supposed to be derived from them by the replacement of one or more hydrogen atoms by other atoms or group of atoms. So these are considered as parent organic compounds.

Hydrocarbons are classified into two main classes:

1. Open chain hydrocarbons
2. Closed chain or cyclic hydrocarbons

Open chain compounds are defined as the compounds in which carbon atoms are linked to one another to form open chains. They may be straight or branched and also referred to as a cyclic or aliphatic compounds. They are also divided into three parts: alkanes, alkenes, and alkynes.

The carbon atoms in which atoms are arranged in such a way that they form rings is known as cyclic or closed chain compounds. The cyclic compounds with the closed ring of carbons atoms only called carbocyclic or homocyclic compounds. They are also called alicyclic compounds (aliphatic cyclic compounds) because here compounds resemble the aliphatic compounds in many ways.open chain hydrocarbons are also divided into cycloalkanes, cycloalkenes, and cycloalkynes.

Nomenclature

Saturated hydrocarbons possess a number of methylene groups joined together to form a cyclic structure, these are named as the polymer of methylene (polymethylene). Methylene groups are indicated by the Greek or Latin prefix. i.e., tri, tetra, penta, hexa, etc for 3, 4, 5, 6……………. Methylene groups.

In IUPAC system saturated monocyclic compounds are named as cycloalkanes (prefix=’cyclo’).

The names of substituents are prefixed before the parent name in alphabetical order if the molecule contains two or more substituents. According to the lowest sum role ring, carbon is numbered to indicate the position of substituents and the position of substituents are indicated by the number of those carbon atoms to which they are attached.

The rule is applied to alkene or alkyne in the case of unsaturated monocyclic hydrocarbons, i.e., they are named as cycloalkane or cycloalkene. But the doubly or triply bonded carbons are considered to occupy the position 1 and 2.

Functional groups such as aldehydes, carboxylic acids, amides, nitriles, etc. cannot be included in the basic name of the cyclic portion of the molecule, instead, they represent a branch from it.

‘Spirocyclic’ is defined as the compound containing two rings with one common carbon atom. It is named using prefix ‘spiro’ before the name of the parent hydrocarbon. We can get the name of parent hydrocarbon by counting the total number of carbon atoms in two rings. The carbon atom on each side of the common carbon atom are written in brackets in descending order.

‘Bicyclic’ is defined as the compound containing two rings with two or more common carbon atoms. It is named using prefix ‘bicyclo’ before the name of parent hydrocarbon. . We can get the name of parent hydrocarbon by counting the total number of carbon atoms present in two rings. The number of carbon atoms in each of bridge joining the two tertiary carbon atoms are written in brackets in descending order.

The ring carbons are numbered to indicate the positions of substituents. By the longest possible path to the second bridgehead, the numbering begins with one of the bridgehead and proceeds, numbering is then continued from this atom through the longer unnumbered path back to the first bridgehead and is completed by the shortest path. The lowest numbers are then assigned to double bonds and substituents only after this rule is taken into consideration.

Industrial source

From the certain areas, in particular, California, the naphtha fraction of the petroleum is found too rich in alicyclic compounds specially five-membered and six-membered. Because of this reason, the alicyclic compounds are also known as naphthenes. The alicyclic compounds which are found in the naphtha fraction of petroleum are cyclohexane, methylcyclohexane, methyl cyclopentane and 1,2-dimethylcyclopentane

Preparations

Preparations of alicyclic compounds from other open chain compounds generally involves two steps:

1. Conversion of some open chain compound or compounds into a cyclic compound. This step is called cyclization.
2. Conversion of the cyclic compound thus obtained into the kind of the desired compound, e.g., conversion of a cyclic alcohol into a cyclic halide or that of a cyclic alkene into cycloalkane.

Some of the methods for preparing alicyclic compounds

1. From dihalides (Freund’s method): Terminal dihalides on treatment with sodium or zinc metal undergo cyclization forming corresponding cycloalkanes.

The terminal halogen derivatives of alkanes in which the two halogen atoms are further apart than the 1,6-positions do not form ring compounds but undergo the Wurtz reaction, i.e., if, in a dihalide, Br(CH₂)_n Br, n > 6, the products are Br(CH₂)_2n Br, Br(CH₂)_3n Br, etc.

2. From salts of dicarboxylic acids (Wislicenus method): Calcium, barium or thorium salt of a suitable dicarboxylic acid (i.e., adipic, pimelic or suberic acid) on distillation forms a cyclic ketone. The cyclic ketone so obtained can be reduced to cycloalkane by Clemmensen reduction. This method is suitable for the preparation of cyclopentane, cyclohexane, and cycloheptane.
3. From esters of dicarboxylic acids (Dieckmann’s method): The reaction is an intramolecular Claisen condensation and provides an important method for synthesis of five, six and seven membered alicyclic compounds is known as Dieckmann’s reaction.

Ester of a suitable dicarboxylic acid on treatment with sodium metal (or sodium ethoxide) undergoes cyclization forming 2-carboalkoxycycloalkanone which on hydrolysis forms 2-carboxycycloalkanone. The decarboxylation of later gives cycloalkanone which on reduction forms cycloalkane.

4. From malonic ester (perkin’s method): This method was suggested by perkin and is a convenient method for the synthesis of alicyclic compounds up to six membered. In this method a terminal Dibromide, Br-(CH₂)_n –Br (where n=2 to 5), is treated with a maonic ester in presence of sodium ethoxide to form a cycloalkane-1,1-dicarboxylic ester which upon hydrolysis followed by the action of heat gives the corresponding cycloalkane.
5. From Diels-Alder reaction: It is the very important reaction for the preparation of cyclic compounds. It involves a reaction between a diene and an unsaturated compound called dienophile. It can be carried out with or without a solvent and may be initiated by heat, light or AlCl₃ as the catalyst. The presence of electron withdrawing groups such as –COOH, -COOR, -NO₂, -CN, etc. activate the rate of the reaction. Dienes are activated by electron donating groups such as –OCH₃, -CH₃, -N(CH₃)₂ , etc. Furthermore, bicyclic products are obtained if the dienophile is cyclic.