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8.2: Polarity, Acidity, and Solubility

Last updated

May 20, 2021
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- 8.1: Substituted Alkanes
- 8.3: The Basics of Determining Mechanism

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Substituted alkanes are polar molecules compared to hydrocarbons, but the intermolecular forces between them are quite unique.

1. induced dipole-induced dipole – temporary dipole moments result in weak attraction, but can accumulate (only available in hydrocarbons)

2. dipole-induced dipole – polar region of one molecule alters the electron distribution in a nonpolar region of another to produce an induced dipole that results in weak attraction

3. dipole-dipole – mutual attraction between the positively-charged region (( $δ^{+}$ ) of one molecule and the negatively-charged region ( $δ^{–}$ of another; generally quite strong

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These data suggest another unusually strong intermolecular force.

4. hydrogen bonding – notice that ethanol has a slightly acidic proton (pKa = 16.2), meaning there is a partial ( $δ^{+}$ on the hydrogen atom. This can interact with the lone pairs ( $δ^{–}$ ) on the oxygen atom to generate a hydrogen bond.

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Hydrogen bonds are usually ~5 kcal/mol and exist whenever there is a hydrogen atom bound to an electronegative atom (ie. N, O, S) and a lone pair. Thus, hydrogen bonding can occur in amines, thiols, and alcohols. Hydrogen bonding is also what makes life possible – the 3-D structures of DNA/RNA, proteins, and carbohydrates all have hydrogen bonding.

The types of intermolecular forces a molecule has will also dictate its solubility in a solvent. Those molecules that can hydrogen bond will dissolve in solvents that can also hydrogen bond – hence, the phrase “like dissolves like.” In the same vein, very nonpolar hydrocarbons will only dissolve in other nonpolar solvents. This happens because the solvation shell will stabilize the molecule. An appropriate choice of solvent is important for an efficient substitution reaction.

protic solvents – contain a labile proton; very polar (ex. MeOH, EtOH, H₂O)
aprotic solvents – do not contain a labile proton; can be polar (THF, DMF, DMSO) or nonpolar (CH₂Cl₂, EtOAc, hexanes)

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