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Basic Principles of Organic Chemistry — Chemistry STD 11 Science — Question

Maharashtra BoardEnglish MediumSTD 11 ScienceChemistryBasic Principles of Organic Chemistry6 Marks
Question
Write a note on homologous series.
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Answer

Homologous series:
A series of compounds of the same family in which each member has the same type of carbon skeleton and functional group, and differs from the next member by a constant difference of one methylene group $(-CH_2-)$ in its molecular and structural formula is called as homologous series.
The individual members of the series are called homologues and they can be represented by a same general formula.
Two successive homologues differ by one $– CH_2 ($methylene$)$ unit $($i.e., molecular weight of each successive member differs by $14$ units$).$
Homologues show similar chemical properties.
Physical properties $($like melting point, boiling point, density, solubility, etc.$)$ of the homologues show a gradual change with increase in the molecular weight of the member.
Note: Consider the homologous series of straight chain aldehydes. The boiling point increases down the series as molecular weight increases.
Name Molecular formula Boiling point
Formaldehyde $HCHO$ $-21^{\circ} C$
Acetaldehyde $CH _3 CHO$ $21^{\circ} C$
Propionaldehyde $C _2 H _5 CHO$ $48^{\circ} C$
Butyraldehyde $C _3 H _7 CHO$ $75^{\circ} C$
Valeraldehyde $C _4 H _9 CHO$ $103^{\circ} C$

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