structures and shapes of hydrocarbons

New Way Chemistry for Hong Kong A-Level Book 3A

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Structures and Shapes of Hydrocarbons

25.125.1 Saturated HydrocarbonsSaturated Hydrocarbons

25.225.2 Unsaturated HydrocarbonsUnsaturated Hydrocarbons

25.325.3 Aromatic HydrocarbonsAromatic Hydrocarbons

Chapter 25Chapter 25


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25.1 Saturated Hydrocarbons (SB p.17)

sp3 Hybridizationsp3 Hybridization


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4


Methane (CH4)


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109.5°

sp3 hydrid orbital


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Ethane (C2H6)

109.5°

109.5°


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Electron Pair Repulsion TheoryElectron Pair Repulsion Theory

• predict the geometry of arrangement of atoms in the molecules

Procedure:1. Focus on the central atom of the molecule

2. Consider all of the valence electron pairs of the central atom, i.e. bond pairs and lone pairs

3. Electron pairs tend to stay as far apart as possible (electronic repulsion between lone pairs is generally greater than that between

bond pairs)

4. The shape of the molecule is referred to the positions of the atoms (the shape has minimum repulsion between the electron pairs)


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e.g. Methane

• tetrahedral orientation

• electron pairs to have the maximum separation of 109.5°


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Check Point 25-1 Check Point 25-1

(a) Draw a three-dimensional structure for propane (C3H8).

Answer


(a)


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(b) How many bonds are there in a molecule of

(i) propane?

(ii) butane?

(iii) methylpropane? Answer

(b) (i) 10 bonds

(ii) 13 bonds

(iii) 13 bonds



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25.2 Unsaturated Hydrocarbons (SB p.22)

sp2 Hybridizationsp2 Hybridization


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13

Ethene (C2H4)



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15

Cyclohexene (C6H10)



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Example 25-1Example 25-1The bond enthalpy of the carbon-carbon double bond in ethene is +612 kJ mol–1 whilst the bond enthalpy of the carbon-carbon single bond in ethane is +348 kJ mol–1. Explain briefly why the bond enthalpy of the carbon-carbon double bond is less than twice that of the carbon-carbon single bond.

Answer


Solution:

A carbon-carbon double bond is composed of a bond and a bond. The bond is formed by the head-on overlap of orbitals. The electrons of the bond are distributed along the internuclear axis of the two bonded atoms. bond is a strong bond and has a high bond enthalpy. On the other hand, a bond is formed by the side-way overlap of orbitals. As the side-way overlap of orbitals is less effective than the head-on overlap of orbitals, a bond is weaker than a bond. Hence, the bond enthalpy of the carbon-carbon double bond (i.e. a bond and a bond) is less than twice that of the carbon-carbon single bond (i.e. a bond)


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(a) State the difference between the bond and the bond in the carbon-carbon double bond.

Answer



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(a) Differences between the bond and the bond in the carbon-carbon double bond:


bond bond

Head-on overlap of the sp2 hydridized orbitals of two carbon atoms

Side-way overlap of the vacant p orbitals of two carbon atoms

The bonding electrons in are localized symmetrically along the internuclear axis of two bonded carbon atoms.

The electrons in the bond appear as two lobes, one above and one below the internuclear axis of the two bonded atoms

Stronger Weaker

Free to rotate Restricted to rotation


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(b) How many and bonds are present in a molecule of

(i) propene?

(ii) but-1-ene?

(iii) but-2-ene?Answer

(b) (i) 8 bonds and 1 bond

(ii) 11 bonds and 1 bond

(iii) 11 bonds and 1 bond



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sp Hybridizationsp Hybridization


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22

Ethyne (C2H2)



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(a) How many and bonds are present in a molecule of

(i) propyne?

(ii) but-1-yne?

(iii) but-2-yne? Answer

(b) (i) 6 bonds and 2 bonds

(ii) 9 bonds and 2 bonds

(iii) 9 bonds and 2 bonds



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(b) What is the hybridization of each carbon atom in ethanenitrile?

1 2

CH3CNAnswer

(b) Carbon 1 is sp3-hybridized,

whereas carbon 2 is sp-hybridized.



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(c) State the bond angles indicated in the compound below:

Answer(c) x: 109.5°

y: 120°

z: 180°



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Benzene (C6H6)


From X-ray crystallography,

• all carbon-carbon bonds are equivalent

• between length of C = C and C – C bond


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electrons are not localized as shown above.

They are delocalized and represented in a better way as follows:


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Example 25-2Example 25-2Classify the following compounds as saturated, unsaturated or aromatic hydrocarbons.

(a)

(b)

Answer

25.3 Aromatic Hydrocarbons (SB p.29)

Solution:

(a) Unsaturated hydrocarbon

(b) Aromatic hydrocarbon


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Example 25-2Example 25-2Classify the following compounds as saturated, unsaturated or aromatic hydrocarbons.

(c)

(d)

(e) Answer


Solution:

(c) Unsaturated hydrocarbon

(d) Saturated hydrocarbon

(e) Saturated hydrocarbon


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Example 25-3Example 25-3Give the approximate values of the indicated bond angles of the following compound:

Answer


Solution:

a = 180° ; b = 109.5° ; c = 120° ; d = 120°


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Which is a stronger bond, a carbon-carbon single bond or a carbon-carbon double bond? Explain your answer.

Answer

A carbon-carbon double bond is stronger than a carbon-

carbon single bond, as the carbon-carbon double bond

contains one bond and one bond whereas the carbon-

carbon single bond contains only one bond.



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The END

structures and shapes of hydrocarbons

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