Consider the molecules H2CCH2, H2CCCH2, and H2CCCCH2. (a) Draw Lewis structures for these molecules. (b) What is the hybridization at each C atom? (c) What type of bond connects the carbon atoms (single, double, etc.)? (d) What are the HCH, CCH, and CCC angles in these molecules? (e) Do all the hydrogen atoms lie in the same plane? (f) A generalized formula for molecules of this type is H2C(C)xCH2, where x is 0, 1, 2, etc. What can be said, if anything, about the relative orientation of the H atoms at the ends of the chain as a function of x?
The correct answer and explanation is :
(a) Lewis Structures:
The Lewis structures for the given molecules are:
- Ethene (H₂C=CH₂)
H H
\ /
C=C
/ \
H H- Allene (H₂C=C=CH₂)
H H
\ /
C=C=C
/ \
H H- Butatriene (H₂C=C=C=CH₂)
H H
\ /
C=C=C=C
/ \
H H(b) Hybridization:
- Ethene (H₂C=CH₂): Each carbon is sp² hybridized.
- Allene (H₂C=C=CH₂): The central carbon is sp hybridized, while the terminal carbons are sp² hybridized.
- Butatriene (H₂C=C=C=CH₂): The two inner carbons are sp hybridized, while the terminal carbons are sp² hybridized.
(c) Type of Bonds:
- Ethene (H₂C=CH₂): The two carbons are connected by a double bond (C=C).
- Allene (H₂C=C=CH₂): The terminal and central carbon atoms are connected by two double bonds (C=C=C).
- Butatriene (H₂C=C=C=CH₂): It has three consecutive double bonds (C=C=C=C).
(d) Bond Angles:
- Ethene (H₂C=CH₂): HCH ≈ 120°, CCH ≈ 120°.
- Allene (H₂C=C=CH₂): HCH ≈ 120°, CCC ≈ 180°.
- Butatriene (H₂C=C=C=CH₂): HCH ≈ 120°, CCC ≈ 180°.
(e) Planarity of Hydrogen Atoms:
- Ethene: All hydrogen atoms lie in the same plane.
- Allene: The two sets of hydrogen atoms on terminal carbons are perpendicular to each other.
- Butatriene: The hydrogens at the terminal carbons are coplanar, but there is a twist as the chain grows longer.
(f) General Formula H₂C(C)xCH₂:
As x increases, alternating hybridization leads to alternating planes. When x is even, the hydrogens at both ends are coplanar. When x is odd, they are in perpendicular planes.
Illustration of Molecules
