Organic Chemistry
7 marks · EasyAnswer the questions about the structure and naming of unbranched alkanes and the C=C bond in alkenes.
Draw the displayed formula of pentane (an unbranched alkane with five carbons) and give its name.
Draw the structure of ethene and label the double bond.
Explain why ethene is classified as unsaturated.
State the type of bond found only in alkenes but not in alkanes.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Displayed formula: CH3CH2CH2CH2CH3 (pentane) · must show unbranched five-carbon chain, correct number of H atoms
(b) Displayed formula: CH2=CH2 with C=C labelled · must show correct C=C double bond, ethene structure
(c) Because ethene contains a carbon–carbon double bond (C=C), it does not contain the maximum number of hydrogen atoms.
(d) carbon–carbon double bond (C=C) or double bond
Show mark scheme and worked solution
Marking points
D1 (a) · 1 mark · Draws correct displayed formula for unbranched pentane
B1 (a) · 1 mark · Correctly names pentane
D1 (b) · 1 mark · Draws correct structure for ethene with double bond
D1 (b) · 1 mark · Labels the C=C bond
B1 (c) · 1 mark · Mentions double bond (C=C) or not maximum H
B1 (c) · 1 mark · Links to unsaturation (not fully hydrogenated / can react)
B1 (d) · 1 mark · States double bond (C=C) is only found in alkenes
Worked solution
a) Pentane: displayed as CH3–CH2–CH2–CH2–CH3 (five carbons, no branches).
b) Ethene: displayed as CH2=CH2. Double bond labelled.
c) Ethene is unsaturated because it contains a carbon–carbon double bond (C=C). This means it does not have the maximum number of hydrogens.
d) The type of bond found only in alkenes is the carbon–carbon double bond (C=C).