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What are Hydrocarbons?

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Alkanes and cycloalkanes are hydrocarbons with no double or triple bond functional groups, depending on whether the carbon atoms of the molecule are organized in chains or rings. Alkenes and alkynes are hydrocarbons with double or triple bonds, respectively. The following mentioned are the rules for naming hydrocarbons. Isomers are chemical molecules with the same molecular formula but distinct structures. The isomers will be discussed in greater depth further down. Also, we’ll go over the homologous series in more detail.

Naming of Hydrocarbons

The International Union of Pure and Applied Chemistry (IUPAC) established the official names or systematic names of organic compounds in 1958, and they are known as IUPAC names or IUPAC nomenclature. The following points should be kept in mind while using the IUPAC method to name hydrocarbons.

  • The number of carbon atoms in a hydrocarbon is represented by the stem below.
One carbon atom Meth
Two carbon atoms Eth
Three carbon atoms Prop
Four carbon atoms But
Five carbon atoms Pent
Six carbon atoms Hex
Seven carbon atoms Hept
Eight carbon atoms Oct
Nine carbon atoms Non
Ten carbon atoms Dec
  • The term ‘ane’ is written after the stem to denote a saturated hydrocarbon with single bonds.
  • The term ‘ene’ is written after the stem to denote an unsaturated hydrocarbon with double bonds.
  • The word ‘yne’ is written after the stem to denote an unsaturated hydrocarbon with triple bonds.

Naming of Saturated Hydrocarbons

  • The naming of CH4– This chemical has one carbon atom, which is denoted by ‘meth’. It’s saturated since it’s made up entirely of single bonds. The ‘ane’ at the end denotes a saturated hydrocarbon. When the ‘meth’ and ‘ane’ are combined, the IUPAC nomenclature for this substance is ‘methane’ (meth+ane=methane). Methane is the IUPAC name as well as the common name for the hydrocarbon CH4.

Structure of Methane CH4

  • The naming of C2H6– The letter ‘eth’ denotes the presence of two carbon atoms in this molecule. It’s saturated since it’s made up entirely of single bonds. The ‘ane’ at the end denotes a saturated hydrocarbon. When ‘eth’ and ‘ane’ are combined, the IUPAC name for this combination is ‘ethane’ (eth+ane=ethane). The IUPAC and common names for the hydrocarbon C2H6 are identical, ethane.

Structure of Ethane C2H6

IUPAC Nomenclature for Branched-Chain Saturated Hydrocarbons

The following rules should be kept in mind when using the IUPAC technique to name saturated hydrocarbons with branching chains.

  1. In the structure of the compound to be named, the longest chain of carbon atoms is located first. The compound is then classified as a derivative of the alkane hydrocarbon, which is the carbon atom chain with the largest length. This is referred to as “parent hydrocarbon.”
  2. The alkyl groups found as side chains (branches) are referred to as substituents and are denoted as methyl (CH3-) and ethyl (C2H5-) respectively.
  3. The longest carbon chain’s carbon atoms are numbered in such a way that the alkyl groups (substituents) receive the lowest possible number.
  4. The number of the carbon atom to which the alkyl group is attached is used to denote its position.
  5. Writing the ‘position and name of alkyl group’ exactly before the name of the ‘parent hydrocarbon’ yields the IUPAC name of the compound.

Naming of Unsaturated Hydrocarbons with Double Bond

  • The naming of C2H4– The letter ‘eth’ denotes the existence of two carbon atoms in this molecule. This hydrocarbon is unsaturated because it has a carbon-carbon double bond. The ‘ene’ at the ending is used to denote a double bond. When the letters ‘eth’ and ‘ene’ are combined, the IUPAC name for this combination is ‘ethene’ (eth+ene=ethene). Ethene has a common name known as ethylene.

Structure of ethene C2H4

  • The naming of C3H6– The letter ‘prop’ denotes the presence of three carbon atoms in this molecule. This hydrocarbon is unsaturated because it has a carbon-carbon double bond. The ‘ene’ at the ending is used to denote a double bond. When the letters ‘prop’ and ‘ene’ are combined, the IUPAC nomenclature for this combination is ‘propene’ (prop+ene=propene). Propane has a common name known as propylene.

Structure of Propene, C3H6

Naming of Unsaturated Hydrocarbons with Triple Bond

  • The naming of C2H2– The letter ‘eth’ denotes the presence of two carbon atoms in this molecule. This hydrocarbon is unsaturated because it has a carbon-carbon triple bond. The ‘yne’ at the ending is used to denote a triple bond. When the letters ‘eth’ and ‘yne’ are combined, the IUPAC name for this combination is ‘ethyne’ (eth+yne=ethyne). Acetylene is the common term for ethyne.

Structure of ethyne, C2H2

  • The naming of C3H4– The letter ‘prop’ denotes the presence of three carbon atoms in this molecule. This hydrocarbon is unsaturated because it has a carbon-carbon triple bond. The ‘yne’ at the ending is used to denote a triple bond. When the letters ‘prop’ and ‘yne’ are combined, the IUPAC nomenclature for this combination is ‘propyne’ (prop+yne=propyne). Propyne has a common name known as methyl-acetylene.

Structure of Propyne, C3H4

What are Isomers?

A molecular formula represents only one substance in inorganic chemistry. For example, HSO stands for sulphuric acid, which is a single compound. However, in organic chemistry, a single molecular formula can be used to represent two or more distinct molecules. This is because the identical carbon atoms in organic molecules can be arranged in a variety of ways to produce distinct structures and thus different compounds. In organic chemistry, for example, the same chemical formula C4H10 can represent two different compounds: normal-butane and iso-butane. The following example will help to clarify this topic.

  • Consider the chemical molecule butane (C4H10). This chemical has four carbon atoms that can be linked in two ways to create two different structures. To begin, all four carbon atoms are connected in a continuous straight chain to form the structure seen below. The compound normal butane, abbreviated as n-butane, is represented by this structure.
  • In the second case, three carbon atoms can be combined in a straight chain and the fourth carbon atom can be joined in a side chain, resulting in the structure illustrated below. Iso-butane is the chemical that has this structure.

Although n-butane and iso-butane have the same chemical formula (C4H10), their structures are distinct. They’re known as isomers. Isomers are chemical molecules with the same molecular formula but distinct structures. In other words, isomers are organic molecules that have the same chemical formula but differ in their carbon atom configurations. Isomers include normal butane and isobutane, which have the same chemical formula but distinct structures or arrangements of carbon atoms. Iso-butane has a branched-chain structure, whereas normal butane has a straight-chain structure. It’s also worth noting that the IUPAC names for n-butane and iso-butane are 2-methylpropane or simply methyl propane.

Isomerism refers to the existence of two or more distinct chemical molecules with the same molecular formula but different structures. Only hydrocarbons with four or more carbon atoms can have two or more distinct configurations of carbon atoms, making isomerism possible. Since only one arrangement of carbon atoms is allowed in hydrocarbons with 1, 2, or 3 carbon atoms per molecule, no isomerism is possible. 

For example, no isomerism is possible in methane, ethane, or propane since they contain only one, two, or three carbon atoms, respectively, and various configurations of carbon atoms are not possible with only 1, 2, or 3 carbon atoms. The chemical butane (C4H10) can have two isomers. Butane’s two isomers have already been discussed. There are three isomers of the chemical pentane (C5H12), and five isomers of the compound hexane (C6H14). The number of potential isomers increases rapidly as the number of carbon atoms in an alkane molecule increases.

What are Homologous Series?

In the same way that all elements with similar electron structures have similar chemical properties and are grouped together in the same periodic table group, all organic compounds with similar structures have similar properties and are grouped together in the same group or series. The organic compounds are then organized in increasing molecular mass order. 

A homologous series is a collection of organic compounds with identical structures and chemical characteristics that differ only in the CH2 group between them. Homologous refers to the different chemical molecules that make up a homologous series. The two neighbouring molecules are clearly separated by one carbon atom and two hydrogen atoms (or CH2 group).

Homologous Series of alkanes- Since all alkanes have identical structures with single covalent bonds and chemical characteristics, they can be classified together in a homologous series. The first five alkanes in the homologous series are listed below.

Alkanes Molecular formula
Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10
Pentane C5H12

The homologous series of alkanes have the general formula CnH2n+2, where n is the number of carbon atoms in one molecule of alkane. One carbon atom makes up the first member of the alkane series. The second alkane series member has two carbon atoms. The third alkane series member has three carbon atoms. The fourth alkane series member has four carbon atoms, while the fifth alkane series member has five carbon atoms.

Homologous Series of alkenes- The homologous series of alkenes has the general formula CnH2n, where n is the number of carbon atoms in one molecule of alkene. The first five alkenes in the homologous series are listed below.

Alkenes Molecular Formula
Ethene C2H4
Propene C3H6
Butene C4H8
Pentene C5H10
Hexene  C6H12

The alkene series begins with the first member, which has two carbon atoms. The second member of the alkene series has three carbon atoms. The third member of the alkene series has four carbon atoms. The fourth members of the alkene series have 5 carbon atoms, whereas the fifth member has 6 carbon atoms.

Homologous Series of alkynes– The homologous series of alkynes has the general formula CnH2n-2, where n is the number of carbon atoms in one molecule of alkyne. The first five alkynes in the homologous sequence are listed below.

Alkynes Molecular Formula
Ethyne C2H2
Propyne C3H4
Butyne C4H6
Pentyne C5H8
Hexyne C6H10

The alkyne series begins with the first member, which has two carbon atoms. The second member of the alkyne series has three carbon atoms. The third member of the alkyne series has four carbon atoms. The fourth member of the alkyne series each has 5 carbon atoms, whereas the fifth member has 6 carbon atoms.

Characteristic of Homologous Series

The following are the properties of the Homologous Series.

  • The chemical characteristics of all substances in a homologous series are similar. All alkane series molecules, such as methane, ethane, propane, and others, undergo substitution reactions with chlorine.
  • A homologous series’ members can all be represented by the same general formula.
  • In their molecular formulas, any two neighbouring homologous differ by one carbon atom and two hydrogen atoms.
  • With increasing molecular mass, members of a homologous series display a steady change in their physical properties.
  • Any two neighbouring homologous molecules have a molecular mass difference of 14u.

Sample Questions

Question 1: Are the isomers of C3H8 possible?

Answer:

For the given compound C3H8, there are 3 carbon atoms. Since only one arrangement of carbon atoms is allowed in hydrocarbon with three carbon atoms, so no isomerism is possible. Hence the isomers of C3H8 are not possible.

Question 2: The given compounds, C3H8, C4H10 are the neighbouring compounds of the same homologous series or not?

Answer:

The two neighboring compounds of the same homologous series are separated by one carbon atom and two hydrogen atoms. Since the given compounds differ by one carbon atom and two hydrogen atoms, i.e. CH2 group. So these compounds are the neighbouring compounds of the same homologous series.

Question 3: The name of the hydrocarbon CH2=CH2 is ethane or not?

Answer:

In the given compound, there are two carbon atoms, so ‘eth’ denotes the existence of two carbon atoms. There is a double bond between carbon carbon atoms, which is indicated by ‘ene’ at the end. So, the name of the given compound should be ethene. But at the end of given name ‘ethane’ there is ‘ane’ which indicated a single bond. Hence ethane is not the name of CH2=CH2.

Question 4: Name a hydrocarbon other than pentane, which has more than three isomers.

Answer:

A hydrocarbon, which has more than three isomers is hexane, C6H14.

Question 5: An alkene has 34 carbon atoms in its molecule. How many hydrogen atom does this molecule have?

Answer:

The general formula CnH2n, where n is the number of carbon atoms in one molecule of alkene. Since it is given that there are 34  carbon atoms, so n=34. Putting the value of n in the above formula,we get C34H2(34), or C34H68. So there are 68 hydrogen atoms in this molecule.



Last Updated : 21 Nov, 2021
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