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Alkyl Groups and Cyclic Hydrocarbons

  • Last Updated : 11 Nov, 2021

Organic compounds are compounds of carbon. Hydrocarbons are organic compounds that contain hydrogen and carbon. Petroleum, also known as crude oil, is the most important natural source of hydrocarbons. Saturated and unsaturated hydrocarbons are the two types of hydrocarbons. When hydrocarbons are arranged in the form of rings, they are called cyclic hydrocarbons. The saturated and unsaturated cyclic hydrocarbons are discussed further below. Also, the alkyl groups which are formed when one hydrogen atom is removed from a saturated hydrocarbon are also discussed below. 

What are Saturated Hydrocarbons?

A saturated hydrocarbon is a hydrocarbon in which all of the carbon atoms are connected by a single bond. Alkanes are another name for saturated hydrocarbon. An alkane is a hydrocarbon with only a single covalent bond between the carbon atoms. In an alkane, there are no double or triple bonds. Methane, ethane, propane, and butane among others are the compounds known as alkanes. 

Saturated hydrocarbons, or alkanes, have the general formula CnH2n+2, where n is the number of carbon atoms in one molecule of the alkane.

What are Unsaturated Hydrocarbons?

Unsaturated hydrocarbons are hydrocarbons in which the carbon atoms are connected by a double or triple bond. A double bond is formed when two carbon atoms share two pairs of electrons, whereas a triple bond is formed when three pairs of electrons are shared between two carbon atoms.

An alkene is an unsaturated hydrocarbon with two carbon atoms connected by a double bond. Alkene has the general formula CnH2n, where n is the number of carbon atoms in one molecule.

The simplest alkene will have two carbon atoms in the molecules since an alkene has only a double bond between two carbon atoms. There does not exist a single-carbon alkene. Ethene is the simplest alkene with two carbon atoms.

An alkyne is an unsaturated hydrocarbon with two carbon atoms connected by a triple bond. Alkyne has the general formula CnH2n-2 where n is the number of carbon atoms in one molecule of the alkyne. 

The simplest alkyne will have two carbon atoms in the molecules since an alkyne has only a triple bond between two carbon atoms. There does not exist a single-carbon alkyne. Ethyne is the simplest alkyne with two carbon atoms. Hydrocarbons that are unsaturated are more reactive than saturated hydrocarbons. So alkenes and alkynes are often more chemically reactive than alkanes.

Alkyl Group

An alkyl group is formed when one hydrogen atom is removed from a saturated hydrocarbon (alkane). When this hydrogen is removed, the stem changes from -ane to -yl, indicating an alkyl group. Alkyl is a functional group of organic chemicals that is made up entirely of carbon and hydrogen atoms organized in a chain. 

The methyl group (CH3-) and the ethyl group (C2H5-) are two examples of alkyl groups. The methyl group -CH3 is formed when one hydrogen is removed from methane, CH4. Similarly, removing one hydrogen atom from ethane (CH3CH3) results in the formation of an ethyl group -CH2CH3. They are not found on their own, instead, they are found attached to other hydrocarbons.  The structural formulas of the methyl group and ethyl group are listed below.

Alkyl Groups

The above free line (-) on the carbon atom of the alkyl group indicates that one valency of carbon atoms in the alkyl group is free. Some functional groups or other alkyl groups can be attached to this free valency of carbon.  The general formula of an alkyl group is CnH2n+1 where n is the number of carbon atoms. 

Cyclic Hydrocarbons

In addition to straight-chain and branched-chain hydrocarbons, some other hydrocarbons have carbon atoms arranged in a ring. Such hydrocarbons are called cyclic hydrocarbons. 

The carbon chain of a cyclic hydrocarbon forms a ring. Saturated or unsaturated cyclic hydrocarbons can exist. Countless organic compounds exist in which a sequence of carbon atoms closes to form a ring rather than being connected in a chain. The saturated cyclic hydrocarbons and the unsaturated cyclic

rocarbons are discussed further below.

Saturated Cyclic Hydrocarbons

Cycloalkanes are saturated hydrocarbons that contain only one ring. A cycloalkane is a cyclic hydrocarbon with single carbon-carbon bonds on all of its carbon atoms. 

They have two fewer hydrogen atoms than an alkane with the same number of carbon atoms, with a general formula of CnH2n (n is an integer greater than 2). Cyclopropane (C3H6) is the simplest cycloalkane, with a three-carbon ring. Its extremely strained geometry makes it unstable and reactive, whereas cyclohexane (C6H12) is the most studied, best known, most important cycloalkane. Cycloalkane rings are commonly represented as polygons, with each corner corresponding to a carbon atom to which the required number of hydrogen atoms are attached to bring the total number of bonds to four.

The structural formula of cyclopropane

Cyclic hydrocarbons’ structural formulas can be represented in a number of ways, two of which are illustrated above. Each atom can be represented by the structure on the left in the diagram above. As shown in the triangle on the right, omitting the element symbols and only showing the shape is simple shorthand. The vertices of the triangle are thought to be carbon atoms. The most basic cycloalkane is cyclopropane and because of its highly strained geometry, it is quite unstable and reactive. Cyclohexane is a cycloalkane with six carbons. Larger cycloalkanes exist, but they are rare.

Unsaturated Cyclic Hydrocarbons

Unsaturated cyclic hydrocarbons are also possible. A cyclic hydrocarbon having at least one carbon-carbon double bond is known as a cycloalkene. 

A cyclic hydrocarbon having at least one carbon-carbon triple bond is known as a cycloalkyne. Benzene is one of the most common cycloalkenes. A benzene molecule has six carbon atoms and six hydrogen atoms. There are three carbon-carbon double bonds and three carbon-carbon single bonds in this molecule. It also includes six single carbon-hydrogen bonds. Below are the structures of cyclohexene (C6H10) and benzene (C6H6).

Unsaturated hydrocarbons

Cyclohexene (C6H10) is a flammable liquid with a distinctive odour. Due to the functionality of the double bond, which allows a wide range of chemistries to be applied and downstream intermediates and products to be derived from it, such as the epoxide, diol, and other useful downstream products, it is a reactive alkene that is used as a building block in several different markets.

Sample Questions

Question 1: Cyclopropane is said to be very reactive. Why

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Answer:

Cyclopropane has an extremely strained geometry which makes it unstable and reactive.

Question 2: What are aromatic hydrocarbons?

Answer: 

The cyclic compounds which contain a single and a double bond at alternate positions and exhibit special properties are known as aromatic hydrocarbons. 

Question 3: Identify which of the given compounds C5H10 and C6H6 is saturated cyclic hydrocarbons

Answer: 

The general formula of saturated cyclic hydrocarbons is CnH2n where n is an integer greater than 2. For C5H10, n=5, and on putting n=5 in the general formula we get, C5H2(5) i.e., C5H10, which matches the given compound. So, it is a saturated cyclic hydrocarbon.

For, C6H6, n=6, and on putting n=6 in the general formula we get, C6H2(6) i.e., C6H12, which do not matches the given compound. So, it is not a saturated cyclic hydrocarbon.

Question 4: Give the formula of the alkyl group for the given compound C3H8.

Answer:

An alkyl group is formed when one hydrogen atom is removed from an alkane. The general formula of an alkyl group is CnH2n+1 where n is the number of carbon atoms. For the given compound C3H8, n=3, so putting n=3 in the general formula of the alkyl group, we get C3H2(3)+1 i.e., C3H7. Hence for the given compound, the formula of the alkyl group is -C3H7.

Question 5: Is the general formula of saturated cyclic hydrocarbons similar to any simple hydrocarbons?

Answer:

The general formula of saturated cyclic hydrocarbons is CnH2n where n is an integer greater than 2. Also, the general formula for an alkene is CnH2n, where n is the number of carbon atoms. So, the general formula of saturated cyclic hydrocarbons is similar to that of alkenes.


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