2.5: Physical Properties of Organic Compounds
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Physical Properties and Intermolecular Forces
The comprehension of intermolecular forces helps us to understand and explain the physical properties of substances, since it is intermolecular forces that account for physical properties such as phases, boiling points, melting points, viscosities, etc. For organic chemistry purposes, we will focus on boiling point (b.p.) and solubility.
Boiling point (b.p):
The boiling point trend of different substances directly correlates with the total intermolecular forces. Generally speaking, the stronger the overall intermolecular force applied to a certain substance, the higher the boiling point of the substance. The boiling point is the temperature at which the liquid phase of the substance vaporizes to become a gas. In order to vaporize a liquid, the intermolecular forces that hold the molecules together must be overcome. The stronger the forces, the more energy is needed to overcome the forces, and a higher temperature is required, thus leading to a higher boiling point.
Between the same kind of intermolecular forces, the strength of the intermolecular interactions increases as the dipole moment of the molecules increases, within a series of compounds of similar molar mass, as shown in Table 2.5.1. Using what we learned previously about predicting relative bond polarities from the electronegativities of the bonded atoms, we can make educated guesses about the relative boiling points of similar molecules.
Table 2.5.1 Relationships between the Dipole Moment and the Boiling Point for Organic Compounds of Similar Molar Mass
| Compound | Molar Mass (g/mol) | IMF | Dipole Moment (D) | Boiling Point (K) |
|---|---|---|---|---|
| C3H6 (cyclopropane) | 42 | London Dispersion forces | 0 | 240 |
| CH3OCH3 (dimethyl ether) | 46 | Dipole-Dipole | 1.3 | 248 |
| CH3CN (acetonitrile) | 41 | Dipole-Dipole | 3.9 | 355 |
Within a series of compounds of similar molar mass, the dispersion forces are at a similar level. However, the three compounds have different molecular polarities. Butane is a non-polar substance that only has dispersion forces, propanal is a polar molecule with both dispersion forces and dipole-dipole forces, and propanol is a polar molecule with an OH bond, so all three types of forces apply to. Therefore, the overall amount of intermolecular forces is strongest for propanol, and weakest for butane, which is in the same order as their boiling points.
Table 2.5.2 Relationships between the intermolecular force and the Boiling Point for Organic Compounds of Similar Molar Mass
| Compound | Molar Mass (g/mol) | IMF | Boiling Point (C) |
|---|---|---|---|
| C4H10 (butane) | 58 | London Dispersion forces | -0.5 |
| CH3CH2CHO (propanal) | 58 | Dipole-Dipole | 48 |
| CH3CH2CH2OH (propanol) | 60 | Hydrogen bond | 97 |
Solubility:
A general rule for solubility is summarized by the expression “like dissolves like”. This means that one substance can dissolve in another with similar polarity, and as a result, with similar intermolecular forces. More specifically:
- Nonpolar substances are usually soluble in nonpolar solvents.
- Polar and ionic substances are usually soluble in polar solvents.
- Polar and nonpolar substances are insoluble to each other.
Water, methanol and ethanol are examples of very polar solvents that can form Hydrogen bonds. Ether, ketone, halide and esters are polar solvents as well, but as they can form hydrogen bonds by themselves, they are not as polar as water or methanol. Non-polar molecules are considered non-polar solvents include hydrocarbons like hexane, benzene, toluene etc.
For some organic compounds, however, it may not be that easy to simply call it polar or non-polar, because part of the compound may be polar, and the another part may be nonpolar. This is often described as hydrophilic or hydrophobic.
- Hydrophobic (hydro, water; phobic: fearing or avoiding) means it does not like water, or is insoluble in water;
- Hydrophilic (hydro, water; philic: loving or seeking) means it likes water, or is soluble in water.
The hydrocarbon part of the organic compound is hydrophobic, because it is nonpolar and therefore does not dissolve in polar water. The functional groups OH, COOH, NH2, etc are polar and are, therefore, hydrophilic. With both hydrophobic and hydrophilic parts present in an organic compound, the overall polarity depends on whichever part is the major one. If the carbon chain is short (1~3 carbons), the hydrophilic effect of the polar group is the major one, so the whole compound is soluble in water; with carbon chains of 4~5 carbons, the hydrophobic effect begins to overcome the hydrophilic effect, and water solubility is lost.
The solubility differences of different alcohols demonstrate this trend clearly; as the length of the carbon chain increases, the solubility of alcohol in water decreases dramatically (Table 2.5.3):
Table 2.5.3: Solubility of different alcohols in water
| Alcohol | Solubility in water (g/100mL) |
|---|---|
| methanol, ethanol, propanol(CH3OH, CH3CH2OH, CH3CH2CH2OH) | miscible (dissolve in all proportions) |
| 1-butanol (CH3CH2CH2CH2OH) | 9 |
| 1-pentanol (CH3CH2CH2CH2CH2OH) | 2.7 |
| 1-octanol (CH3CH2CH2CH2CH2CH2CH2CH2OH) | 0.06 |
For organic compounds that are water insoluble, they can sometimes be converted to the “salt derivative” via a proper reaction, and thus can become water soluble. This method is used commonly in labs for the separation of organic compounds.
Example:
Applying acid-base reactions is the most common way to achieve such purposes. As shown in the above example, by adding a strong base to the benzoic acid, an acid-base reaction occurs and benzoic acid is converted to its salt, sodium benzoate, which is water soluble (because of the ion-dipole force as we learned earlier). The benzoic acid can, therefore, be brought into water (aqueous) phase, and separated from other organic compounds that do not have similar properties.
Of the two vitamins A and C, one is hydrophilic and water-soluble while the other is hydrophobic and fat-soluble. Which is which?
- Answer
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Vitamin C is water-soluble (hydrophilic); vitamin A is fat-soluble (hydrophilic).