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Question
Name the following organic compound:
The first homologue whose general formula is CnH2n.
Solution
Ethene
Explanation:
CnH2n is the general formula for the homologous series of alkenes. The first member of the alkene family is the compound having two carbon atoms which are known as ethene. Methene is not possible as at least two carbons are required for the formation of the double bond.
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RELATED QUESTIONS
What is a homologous series?
What is the next higher homologue of methanol (CH3OH)?
Give the molecular formula of one homologue of each of the following:
C2H6
By how many carbon atoms and hydrogen atoms do any two adjacent homologues differ?
What is a homologous series? Explain with an example.
Study the different conclusions drawn by students of a class on the basis of observations of preserved/available specimens of plants and animals.
I. Potato and sweet potato are analogous organs in plants.
II. Wings of insects and wings of birds are homologous organs in animals.
III. Wings of insects and wings of bats are analogous organs in animals.
IV. Thorns of citrus and tendrils of cucurbita are analogous organs in plants.
The correct conclusions are:
(A) I, and II
(B) II and IV
(C) I and III
(D) III and IV
Two statements are given - one labeled Assertion (A) and the other labeled Reason (R).
Assertion (A): In a homologous series of alcohols, the formula for the second member is C2H5OH and the third member is C3H7OH.
Reason (R): The difference between the molecular masses of the two consecutive members of a homologous series is 144.
As one ascends in any homologous series, physical properties change gradually.
Complete the following table for the homologous series of alkanes.
| Name | Molecular formula | Condensed structural formula | Number of carbon atom | Number of -CH2- units | Boiling point °C |
| Methane | CH4 | CH4 | 1 | 1 | -162 |
| Ethane | C2H6 | CH3–CH3 | 2 | 2 | -88.5 |
| Propane | C3H8 | CH3–CH2–CH3 | 3 | 3 | -42 |
| Butane | C4H10 | CH3–CH2–CH2–CH3 | ______ | ______ | 0 |
| Pentane | C5H12 | CH3–CH2–CH2–CH2–CH3 | ______ | ______ | 36 |
| Hexane | C6H14 | CH3–CH2–CH2–CH2–CH2–CH3 | ______ | ______ | 69 |
C3H8 belongs to the homologous series of ______.
