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Questions and Answers of Organic Chemistry

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1.Unsaturated Hydrocarbon:
Problem 1:
Write down the common names of the following compounds:

(i) CH₂CH₂CH₂CH=CH₂ (ii) CH₃CH=CHCH₂CH₃.
Solution:
(i) CH₂CH₂CH₂CH=CH₂
Parent alkane is pentane
Thus the name of the Compound is 1-pentylene or α-pentylene.
(ii) CH₃CH=CHCH₂CH₃
Parent alkane is pentane
Thus the name of the Compound is 2-pentylene or β-pentylene
Problem 2:
What are the names of the following compounds in the IUPAC system ?
(i) CH₃ - CH₂ - CH = CH₂ (ii) C(CH₃)₂ = CH₂
(iii) CH₃ - CH = C(CH₃) - CH₂ - CH₃

(iv) CH₂ = C(C₂H₅) - CH(CH₃)₂

Solution:
I.U.P.A.C. System of Nomenclature
but-1-ene
I.U.P.A.C. System of Nomenclature
2-methyl-prop-1-ene
I.U.P.A.C. System of Nomenclature
3 - methylpent - 2 - ene
I.U.P.A.C. System of Nomenclature
2 - ethyl - 3 - methylbut - 1 - ene
Problem 3:
Arrange the following Alkenes in order of increasing stability and give your reasons.
(i) Me₂C=CH₂, (ii) cis-MeCH=CHMe, (iii) trans-MeCH=CHMe (iv) MeCH₂CH=CH₂ 
Answer:

MeCH₂CH=CH₂(but-1-ene)ㄑcis-MeCH=CHMe(cis but-2-ene)ㄑtrans-MeCH=CHMe(trans but-2-ene) ㄑMe₂C=CH₂(isobutene)
Problem 4:
Which of the following carbocation is more stable?
(i) CH₃CH₂ (ii) CH₃(CH₃)CH (iii) CH₃(CH₃)CCH₃.
Answer:
Explanation for the order of the stability of these carbocation by hyperconjugation.
The number of hydrogen atoms available for hyperconjugation is 3 for (i), 6 for (ii) and 9 for (ii). Consequently, (iii) would be expected to be the most stable.
Thus the stability order is (iii) 〉(ii) 〉(i)