Question

Which of the following solvents are polar: (a) acetonitrile; (b) water; (c) acetic acid; (d) fluorosulfonic acid; (e) dichloromethane; (f) bromine trifluoride; (g) hexane; (h) THF; (i) DMF; (j) liquid sulfur dioxide; (k) benzene?

Short answer

The polar solvents are: acetonitrile, water, acetic acid, fluorosulfonic acid, dichloromethane, bromine trifluoride, THF, DMF, and liquid sulfur dioxide.

Step-by-step solution

Understand Polar Solvents

Polar solvents have significant positive and negative ends due to differences in electronegativity, resulting in a dipole moment. This property allows them to dissolve polar substances and ionic compounds by stabilizing charges or dipoles within the solute.

Evaluating Acetonitrile

Acetonitrile (336e) is polar due to the large electron negativity difference between carbon and nitrogen in the triple bond, allowing it to dissolve many polar compounds.

Evaluating Water

Water (69f2fe) is one of the most polar substances due to the significant electronegativity difference between hydrogen and oxygen and its bent molecular shape.

Evaluating Acetic Acid

Acetic acid (36895) is polar since it has a carboxylic acid group, which is polar because of the different electronegativities within the hydroxyl and carbonyl groups.

Evaluating Fluorosulfonic Acid

Fluorosulfonic acid (FSO3H) is polar because its molecular structure contains very electronegative fluorine and oxygen atoms, creating a strong dipole.

Evaluating Dichloromethane

Dichloromethane (362c2c) is polar due to the presence of polar C-Cl bonds. However, it is less polar than water due to its symmetrical geometry.

Evaluating Bromine Trifluoride

Bromine trifluoride (22fadf) is polar because of its asymmetric T-shaped geometry and the presence of highly electronegative fluorine atoms.

Evaluating Hexane

Hexane (688
0c) is non-polar as it consists entirely of C-C and C-H bonds, which have negligible electronegativity differences, and its structure is symmetrical.

Evaluating THF

Tetrahydrofuran (THF, 38866) is polar due to the presence of an oxygen atom with lone pairs, generating a significant dipole moment.

Evaluating DMF

Dimethylformamide (DMF, 38888) is polar as it contains a highly polar amide bond and exhibits substantial difference in electronegativity between its constituents.

Evaluating Liquid Sulfur Dioxide

Liquid sulfur dioxide (868f) is polar due to the presence of an S=O bond that contributes to a substantial dipole moment.

Evaluating Benzene

Benzene (888e) is non-polar because despite having polar C-H bonds, the molecule's symmetrical ring structure cancels out any dipole moments.

Key concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons. In polar solvents, significant differences in electronegativity between atoms can lead to molecules having positive and negative ends, known as a dipole. This difference is crucial in determining if a substance will be a polar solvent.
For example, in water (H₂O), oxygen is much more electronegative than hydrogen. This creates a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms, making water a highly polar solvent. Similarly, molecules like acetonitrile and dichloromethane exhibit polar characteristics due to electronegativity differences between their constituent atoms.
Key points about electronegativity:

• Greater differences in electronegativity between atoms usually increase the molecule's polarity.
• Electronegativity values are higher for nonmetals such as oxygen, nitrogen, and fluorine.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It determines the overall polarity of a molecule. If the structure of a solvent allows for a net dipole moment, the solvent will be polar and can dissolve other polar substances.
Water has a high dipole moment due to its bent molecular structure and distinct electronegativity difference. The hydrogen atoms are positioned in a manner that does not cancel out the dipole moment created by the oxygen atom. On the other hand, dichloromethane, though not as polar as water, has a detectable dipole moment due to the spatial arrangement of its chlorine atoms.
Notable points about dipole moment:

• Molecules with higher dipole moments are generally more polar.
• Symmetrical structures can lead to cancellation of dipole moments even if the individual bonds are polar.

Solvent Polarity

Solvent polarity refers to the ability of a solvent to stabilize charges or dissolve polar substances. Polar solvents have unique characteristics that allow them to interact with ionic and other polar compounds to facilitate dissolution.
This property is significantly influenced by electronegativity and dipole moments. For instance, solvents like DMF and THF are able to dissolve various polar substances and ionic compounds due to their polar nature. They provide a medium where ionic and polar interactions can take place, thus enhancing solubility. Water, known universally for its solvent capabilities, demonstrates how polarity can play a crucial role in dissolving a wide range of compounds.
Important aspects of solvent polarity:

• A solvent's polarity is a decisive factor in determining its solubility profile.
• Polar solvents can stabilize charged species, enhancing their dissolution capabilities.

Molecular Geometry

Molecular geometry plays a key role in determining whether a molecule is polar or non-polar. It refers to the three-dimensional shape of a molecule and influences how molecular polarities are distributed.
Bromine trifluoride is a classic example where its T-shaped geometry results in a net dipole moment, making it polar. This occurs despite the molecule having polar bonds, reinforcing the importance of geometry in determining overall polarity. In contrast, a molecule like benzene is non-polar due to its symmetrical planar ring structure, allowing its polar bonds to cancel each other's effects.
Core insights into molecular geometry:

• The 3D structure can enhance or cancel out the effects of individual bond polarities.
• Asymmetrical molecules are more likely to be polar due to the imbalance of charge distribution.