ASChemistryCIEExam Questions13. An Introduction to AS Level Organic Chemistry13.3 Shapes of Organic Molecules; σ & π Bonds

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First exams 2025

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Shapes of Organic Molecules; σ & π Bonds (CIE AS Chemistry)

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13.3 Shapes of Organic Molecules; σ & π Bonds

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13.3 Shapes of Organic Molecules; σ & π Bonds

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1a1 mark
Aliphatic compounds can be organised into three different categories depending on their structure. 

 

One of the categories is straight-chain. Name the other two categories of aliphatic compounds. 

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1b3 marks

Complete Table 1.1 to show the type and number of bonds in hybridised bonds.

 
Table 1.1
 
Hybridisation Number of σ bonds Number of π bonds
sp3     
sp2     
sp     

 

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1c4 marks

Four compounds are shown in Fig 1.1.

 
UdwnRtn2_3-1-4c-e-four-molecules
 
Fig 1.1
 

Complete Table 1.2 to name the shapes and bond angles of the four planar molecules.

 
Table 1.2
 
Molecule Shape Bond angle / o
Carbon dioxide  Linear  
Water   104.5
Boron trifluoride   120
Cis-platin Square planar  
 

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1a3 marks

Explain how the concept of hybridisation can be used to explain the triple bond present in propyne.

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1b1 mark

Consider the molecule shown in Fig. 1.1 which contains both sigma and pi bonds.

 
q4b_14-2_medium_ib_hl_sq
 
Fig. 1.1
 

State the number of carbon atoms that exhibit sp2 hybridisation in this molecule.

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1c3 marks

Complete Table 1.1 to show the hybridisation of the nitrogen atoms in NF4+, N2H2 and N2H4.

 
 
Table 1.1
  
   NF4+ N2H2 N2H4
Hybridisation      

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2a2 marks

This question is about the structure and reactions of organic compounds.

Fig. 2.1 shows the displayed formula of a type of fuel known as gasoline. 

 
gasoline
 
Fig. 2.1
  
i)
State the IUPAC name of gasoline.
 
[1]
 
ii)
Give the general formula for the homologous series to which gasoline belongs.
 
[1]

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2b6 marks

The gasoline shown in Fig. 2.1 can react with chlorine in the presence of UV light to form various chlorinated products.

 
i)
State the type of organic mechanism used in this reaction.
 
[1]
 
ii)
Draw the skeletal formula of the four monochlorinated isomers formed when the gasoline in Fig. 2.1 reacts with chlorine. 
 
[4]
 
iii)
State the type of isomerism shown by the isomers in (ii).
 
[1]

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2c4 marks

There are two possible reactions of 1-bromopropane with sodium hydroxide.

 

Fig. 2.2 shows the mechanism for the reaction of aqueous sodium hydroxide with 1-bromopropane.

 

oh--substitution-of-1-bromopropane
 
Fig. 2.2
 
i)
State the role of the aqueous hydroxide ion in this reaction.
 
[1]
 
ii)
Name compound X. 
 
[1]
 
iii)
In different reaction conditions, an alternative reaction occurs between the hydroxide ion and 1-bromompropane to form propene and a small molecule.
 
State the reaction conditions and name the type of mechanism used in this alternative reaction.
 
[2]

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2d4 marks

Pentanal reacts with hydrogen cyanide ions to form 2-hydroxyhexanenitrile as outlined in Fig. 2.3. 

 
pentanal-to-2-hydroxyhexanenitrile
 
Fig. 2.3
 
i)
In terms of orbitals and bonding, explain why the shape of the carbonyl carbon highlighted in Fig. 2.3 can be described as trigonal planar.
 
[3]
 
ii)
State the shape and bond angle adopted by the carbonyl carbon after the reaction with hydrogen cyanide.
 
[1]

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3a2 marks

Hybridisation of the s and p orbitals can be used to explain the shapes observed in molecules.

Explain why cyclohexane C6H12 has a puckered, non-planar shape by making reference to the C–C–C bond angles and the type of hybridisation of carbon in the molecule. 

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3b4 marks

Urea, CO(NH2)2, is present in solution in animal urine. 

 

Describe the hybridisation of C and N in the molecule, giving approximate bond angles.

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3c4 marks

Describe the hybridisation of the carbon atom in methane and explain how the concept of hybridisation can be used to explain the shape of the methane molecule.

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3d1 mark

A molecule of ethanol is shown in Fig. 3.1.

labelled-ethanol

Fig. 3.1

Deduce the hybridisation of the carbon atom marked in the diagram below.

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1a2 marks

Ibuprofen is a common non-steroidal anti-inflammatory drug (NSAID). It contains a benzene ring and a carboxylic acid at the end of one of the branches as shown in Fig 1.1.

ibuprofen-structure-ib-hl-sq-h-14-2-q1a

Fig 1.1

Deduce the number of sigma (σ) and pi (π) bonds in ibuprofen.

 
sigma ..............................
 
pi ..............................

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1b2 marks

The ibuprofen molecule contains both sp2 and sp3 hybridised carbon atoms.

 

State the number of sp2 and sp3 hybridised carbon atoms in a molecule of ibuprofen.

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1c2 marks

Explain why the benzene ring is a planar hexagon.

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