Draw the Lewis structure of acetone, CH3COCH3 and chloroform, CHCl3 using correct bond angle relationships; include partial charges on each atom (if any). Show how these two molecules position themselves when next to each other and show the attraction using a dotted line. Answer questions? a) List the primary and secondary intermolecular forces (in the order of significance or strength) in acetone: ____ , _____ b) Is acetone a polar or nonpolar molecule? ___ , c) Which is more polar, acetone or chloroform? ____ . d) Do you predict the ?Hsln for mixing acetone with chloroform to be positive (>0), negative
The correct answer and explanation is:
Lewis Structures and Intermolecular Interactions
- Acetone (CH3COCH3CH_3COCH_3):
- The central carbon (C=O) is double-bonded to oxygen and single-bonded to two methyl groups (CH3CH_3).
- Oxygen is more electronegative, bearing a partial negative charge (δ−\delta^-), while the carbonyl carbon bears a partial positive charge (δ+\delta^+).
- Chloroform (CHCl3CHCl_3):
- The central carbon is bonded to one hydrogen atom and three chlorine atoms.
- Chlorine atoms are more electronegative than carbon and hydrogen, bearing partial negative charges (δ−\delta^-), while carbon bears a partial positive charge (δ+\delta^+).
- Positioning of Acetone and Chloroform:
- Acetone and chloroform are positioned such that the partially negative oxygen atom in acetone (δ−\delta^-) is attracted to the partially positive hydrogen atom (δ+\delta^+) in chloroform. This forms a hydrogen bond, which is represented by a dotted line.
Answering the Questions:
a) Primary and Secondary Intermolecular Forces in Acetone:
- Primary: Dipole-dipole interactions (due to the polar C=OC=O bond).
- Secondary: London dispersion forces (weak forces due to temporary dipoles).
b) Is acetone polar or nonpolar?
- Polar, because of the C=OC=O bond and asymmetric geometry.
c) Which is more polar, acetone or chloroform?
- Acetone is more polar because the C=OC=O bond contributes a stronger dipole moment compared to chloroform’s dipole-dipole interactions from its asymmetric distribution of C−HC-H and C−ClC-Cl bonds.
d) Prediction of ΔHsln\Delta H_{\text{sln}} for Acetone-Chloroform Mixing and Explanation:
- Negative (<0< 0), because acetone and chloroform form hydrogen bonds when mixed. These strong interactions release energy, making the mixing process exothermic.
Explanation:
When acetone and chloroform mix, they form strong hydrogen bonds between the partially negative oxygen (δ−\delta^-) in acetone and the partially positive hydrogen (δ+\delta^+) in chloroform. These hydrogen bonds represent a favorable interaction that lowers the overall energy of the system. Intermolecular forces in acetone are dominated by dipole-dipole interactions from the polar C=OC=O group, while chloroform’s intermolecular forces primarily stem from dipole-dipole interactions and weak hydrogen bonding due to the C−HC-H bond.
Acetone, being more polar due to its C=OC=O group, forms stronger dipole interactions compared to chloroform. However, when mixed, the ΔHsln\Delta H_{\text{sln}} is negative because the energy released from forming hydrogen bonds between acetone and chloroform compensates for the energy required to overcome their original intermolecular forces. This results in a net exothermic process.