Draw the Lewis structure for the molecule CHCCH3. How many sigma and pi bonds does it contain?
The correct answer and explanation is:
To draw the Lewis structure for CHCCH₃ (also written as CH≡C–CH₃, known as propyne), follow these steps:
✅ Lewis Structure of CHCCH₃ (Propyne)
- Determine total valence electrons:
- Carbon (C): 4 valence electrons × 3 atoms = 12
- Hydrogen (H): 1 valence electron × 4 atoms = 4
- Total = 12 + 4 = 16 valence electrons
- Skeleton structure:
CH≡C–CH₃- The first C (leftmost) is bonded to one H.
- That C is triple bonded to the second C.
- The second C is single bonded to a third C (the CH₃ group).
- The third C is bonded to 3 H atoms.
- Place bonds:
- C–H (left) = 1 sigma bond
- C≡C = 1 sigma + 2 pi bonds
- C–C (middle to methyl) = 1 sigma bond
- C–H × 3 (on methyl) = 3 sigma bonds
- Assign electrons and check octets:
All carbon atoms follow the octet rule, and all hydrogen atoms have 2 electrons in their bond.
✅ Sigma (σ) and Pi (π) Bonds Count
- Sigma (σ) bonds:
- 1 from C–H (left)
- 1 from C≡C
- 1 from C–C (single bond to CH₃)
- 3 from C–H (on methyl)
- Total sigma bonds = 6
- Pi (π) bonds:
- 2 from the triple bond between the two carbon atoms
- Total pi bonds = 2
✅ Final Answer:
- Sigma (σ) bonds: 6
- Pi (π) bonds: 2
🧠 Explanation (300 words)
To understand the bonding in CHCCH₃ (propyne), we begin by interpreting its structural formula. The molecule consists of three carbon atoms and four hydrogen atoms. The first two carbon atoms are connected by a triple bond, and the second carbon is connected to a methyl group (CH₃). The molecule looks like this:
H–C≡C–CH₃
In Lewis structures, each bond is made of electrons shared between atoms. A sigma (σ) bond is the first bond formed between any two atoms and involves head-on orbital overlap. Pi (π) bonds arise from the sideways overlap of p-orbitals and only exist when there are double or triple bonds.
In a triple bond (as seen between the first and second carbon), there is one sigma bond and two pi bonds. The sigma bond gives the primary linkage, and the pi bonds contribute additional strength and rigidity to the bond. Between the second and third carbon atoms (C–C single bond), there is just one sigma bond. The hydrogen atoms bonded to carbon contribute only sigma bonds.
So, adding all:
- 1 sigma (C–H on first carbon)
- 1 sigma (from the C≡C triple bond)
- 2 pi (from the C≡C triple bond)
- 1 sigma (C–C single bond to methyl group)
- 3 sigma (C–H bonds on the methyl group)
Hence, the molecule contains 6 sigma bonds and 2 pi bonds. These bond counts help in predicting the molecule’s shape, reactivity, and properties like boiling point or bond strength.