Examkin

Question

Difficulty: MediumTetrahedral Carbon, Bonding, and Hybridization

What is the total number of sigma (σ\sigma) bonds in a single molecule of prop-2-enal (acrolein, CH2=CH−CHO\text{CH}_2=\text{CH}-\text{CHO})?

Answer: 7 / seven / 7 sigma bonds / 7 sigma

Answer

7
Expanding prop-2-enal (CH2=CH−CHO\text{CH}_2=\text{CH}-\text{CHO}) reveals four C−H\text{C}-\text{H} single bonds, one C−C\text{C}-\text{C} single bond, one C=C\text{C}=\text{C} double bond (composed of one σ\sigma and one π\pi bond), and one C=O\text{C}=\text{O} double bond (composed of one σ\sigma and one π\pi bond). Summing all head-on orbital overlaps yields a total of 7 sigma (σ\sigma) bonds.

Step-by-Step Solution

1
Draw the expanded structural formula of prop-2-enal.
The expanded formula showing all individual atoms and bonds is H2C=CH−C(=O)H\text{H}_2\text{C}=\text{CH}-\text{C}(=\text{O})\text{H}.
Expanding the structural formula ensures that all implicit single bonds, double bonds, and hydrogen attachments are explicitly visible for counting.
2
Count all carbon-hydrogen (C−H\text{C}-\text{H}) single sigma bonds.
There are 2 C−H\text{C}-\text{H} bonds on the terminal alkene carbon, 1 C−H\text{C}-\text{H} bond on the central alkene carbon, and 1 C−H\text{C}-\text{H} bond on the aldehyde carbon, giving a total of 4 C−H\text{C}-\text{H} σ\sigma bonds.
Every single covalent bond formed with hydrogen involves head-on sp2−ssp^2-s orbital overlap and constitutes one σ\sigma bond.
3
Count the sigma bonds among the carbon-carbon and carbon-oxygen links.
The C=C\text{C}=\text{C} double bond contains 1 σ\sigma bond, the C−C\text{C}-\text{C} single bond contains 1 σ\sigma bond, and the C=O\text{C}=\text{O} double bond contains 1 σ\sigma bond, giving 3 heavy-atom σ\sigma bonds.
Every covalent bond—whether single, double, or triple—contains exactly one σ\sigma bond resulting from axial head-on orbital overlap.
4
Sum the total number of sigma bonds.
4 (C-H \sigma bonds)+3 (C-C and C-O \sigma bonds)=7 \sigma bonds4\text{ (C-H \sigma\ bonds)} + 3\text{ (C-C and C-O \sigma\ bonds)} = 7\text{ \sigma\ bonds}.
Adding all localized head-on orbital overlaps gives the total count of sigma bonds in the molecule.

Key Concept

Determination of sigma (σ\sigma) and pi (π\pi) bond counts in organic structures
Rate this question

Topics

Tetrahedral Carbon, Bonding, and Hybridization

More questions from this topic