What are Alkanes and Cycloalkanes?
Alkanes are saturated organic compounds which are made up of C and H atoms. Alkanes have the general formula of CnH2n+2. Alkane contains sp3 hybridized carbon atoms with four sigmas (σ) bonds & every hydrogen atom is connected with one of the carbon atoms. The size of the alkane could be determined by the number of carbon atoms located in it.
Cycloalkanes can be defined as cyclic hydrocarbons in which the carbon atoms in the compound are located in the shape of a ring. These cycloalkanes are also saturated organic compounds (i,e) all the carbon atoms present in the ring are singly connected to the nearby atoms. Cycloalkanes have two fewer H atoms than alkane. Consider the example, the molecular formula of cyclohexane is C6H12 whereas the molecular formula of hexane is C6H14. Cycloalkanes are the isomers of the alkenes. Even though the alkenes and cycloalkanes have an equivalent general formula, cycloalkanes are saturated and have only single bonds.
Subdivisions of Alkanes
Alkanes could be categorized as follows:
- Linear alkanes.
- Branched alkanes.
- Cyclic alkanes.
The straight-chain alkanes having no branching are termed linear alkanes. For these alkanes, the prefix ‘n’ is given. Consider the example, butane is a linear alkane. The name butane could also be given as n-butane. The structure of n-butane could be depicted as follows:
The alkanes with one or more branching in their structure are termed branched alkanes. Due to this branching various isomerism can exist for a specific type of alkane. These isomers can be termed constitutional isomers since they contain equivalent molecular formulas & different structural formulas. For example, n-pentane can exist as neopentane & isopentane. The isomers of pentane could be depicted as follows:
The name of the cyclic alkanes starts with the prefix cyclo- to differentiate them from alkanes. Cycloalkanes consist of one or more rings. A well-known example of cycloalkane is cyclohexane. The structure of cyclohexane could be depicted as follows:
IUPAC nomenclature of alkanes and cycloalkanes
- For unbranched alkanes, the name could be given as that the prefix depicting the number of carbon atoms & the ending ane will depict that the molecule is saturated.
Consider the example,
CH3-CH3, this alkane could be named ethane.
- In the case of branched alkanes, the longest chain of the carbon atoms is taken as the parent chain & their name is taken as parent name of the alkane. Each substituent present on the parent chain is given with the name and number. The substituent should be given with lower number. The number depict that the carbon atoms of the parent chain to which the substituent is connected. For example,
The name of this alkane is 2-methyl butane.
- When there are two or more equivalent substituents are located in the chain, then number the parent chain in a way that provide the small number for the substituent which encountered first. The number of times the substituent exists could be depicted by the prefix di, tri, tetra & so on. The position numbers are separated by using commas. Consider the example,
Here, the alkane contains two identical substituents. Hence the name of the alkane could be given as 2,4-dimethyl hexane.
- When there are two or more different substituent are located in the chain, then the substituents are listed by using alphabetical order and the chain should be numbered from the end which gives the small number to the substituent which encountered first. When there are two substituents in same place on the opposite ends of the parent chain, then the substituent contains lower alphabetical order must be given with the lower number. Consider the example,
The name of the molecule should be given as 3-ethyl-5-methyl heptane.
- Inorder to name the cycloalkanes, the prefix ‘cyclo’ should be given to the corresponding opern chain hydrocarbon and every substituent present on the ring should be given with the name.
- When the molecule having only one substituent, then there is no requirement to give it a number. Consider the example,
The name of this compound is methyl cyclopentane.
- When there are 2 substituents, then the ring should be numbered in a way that starting with the substituent with lower alphabetical order. Consider the example,
Here, the name of the compound is 1-ethyl-2-methylcyclohexane.
Physical properties of alkanes and cycloalkanes
- The very essential property of alkanes & cycloalkanes is their lact of polarity. Because the electronegativity difference of hydrogen & carbon is very small (0.4). Therefore the alkanes are classified as non-polar compounds. Due to the lack of polarity, there is only weak interactions are exist between their molecule.
- The boiling point of alkanes & cycloalkanes are smaller whern compared to other compounds having equivalent molecular weight. As the molecular weight increases, the melting and boiling point of alkanes are also increases.
- Methane exist as gas at room temperature & atmospheric pressure. It can be converted to liquid on cooling, and also converted to solid on further cooling. Methane could be occur as liquid or solid based on th occurrence of forces of attraction between the molecules. The force which exist between the molecules of methane is dispersion force and this force is account for the smaller molecular weight & non polar compounds like methane can be liquiefied.
- Similar to the boiling point, the melting point of alkanes & cycloalkanes are also increases with increase in molecular weight.
Characteristics reactions of alkanes and cycloalkanes
The very essential chemical property of alkanes & cycloalkanes is their inertness. Since they are nonpolar compounds having only sigma bonds, they are quite unreactive towards many reagents. The very essential reaction of an alkane is the combustion of methane to produce water and carbon dioxide.
Context and Applications
This topic is significant for both undergraduate and postgraduate courses, especially for Bachelors and Masters in Chemistry.
Question 1: Among the statements regarding alkanes, which is incorrect?
- Alkane is an acyclic saturated hydrocarbon
- C-C bonds are single in alkanes
- The general formula of alkane is CnH2n
- The other name of alkane is paraffine
Answer: Option 3 is correct.
Explanation: The incorrect statement is option 3. Because the general formula of an alkane is CnH2n+2. They contain two additional H atoms compared to others.
Question 2: The number of H atoms present in the alkane containing six carbon atoms is _______.
Answer: Option 1 is correct.
Explanation: The general formula of an alkane is CnH2n+2, if n=6 then the number of hydrogen atoms is (2x6)+2=14.
Question 3: Among the given cycloalkanes, which is having the highest ring strain?
Answer: Option 3 is correct.
Explanation: Among the given compounds, the compound having the highest ring strain is cyclopropane. Because in the case of cyclopropane, the carbon atoms are located in the shape of a triangle and form C-C-C.
Question 4: Among the statements regarding cyclohexane, which one is incorrect?
- It act as an organic solvent
- It is a hydrophilic compound
- It is non polar in nature
- It is used for various applications
Answer: Option 2 is correct.
Explanation: The incorrect statement is option 2. Because cyclohexane is a hydrophobic compound and is nonpolar in nature.
Question 5: Among the given alkanes, the smallest alkane that forms a ring structure is ________.
Answer: Option 2 is correct.
Explanation: Among the given alkanes, cyclopropane is the only smallest alkane that could produce a ring structure with a C-C bond on the removal of an H atom.
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