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2.6.1 Intermolecular Forces
In organic chemistry, understanding the physical properties of organic compounds, such as boiling point (bp), molecular polarity, and solubility, is very important. It gives us useful information on the correct handling of a substance. These physical properties are essentially determined by the intermolecular forces involved.intermolecular forcesare the attractive forcebetween molecules and holding the molecules together; it is an electric force in nature. We focus on three types of intermolecular forces: dispersion forces, dipole-dipole forces, and hydrogen bonding.
dispersion forces
Dispersion forces (also called London forces) result from the instantaneous and induced dipole of molecules. For non-polar molecules, the constant shifting and distortion of the electron density leads at a certain point in time to a weak and short-lived dipole, called the instantaneous dipole. These temporary dipoles induce electrons in a neighboring molecule to also distort and develop their own transient dipole, which is the induced dipole. Ultimately, all non-polar molecules are attracted across the two types of temporary dipoles, as shown in Fig.Abb. 2.6a. The dispersive force is inherently weak and is the weakest intermolecular force. However, since it applies to all types of molecules (it is the only intermolecular force for non-polar molecules), dispersion forces are also the most fundamental intermolecular force.
The size of the dispersion forces depends on two factors:
- the relativepolarizabilityof electrons. The simple understanding of polarizability is the ease with which electrons are distorted. With larger atoms, there are more electrons in a larger space, so the electrons are more loosely held and more easily polarized, so the scattering force is stronger. In general, the greater the molar mass of the molecule, the stronger the dispersive power.
- the relativesurfaceof the molecule. Molecules with longer, flat or cylindrical shapes have a larger surface area compared to bulky and branched molecules and therefore have stronger dispersing power. Considering the two constitutional isomers of C4H10(Section 2.1.2), butane and isobutane, the dispersing power of butane is stronger than that of isobutane.
dipole-dipole force
With polar molecules, the molecules are attracted due to a permanent dipole, and this type of attraction is called the dipole-dipole force. As shown below in the electrostatic potential map of acetone, one end of acetone has a partial negative charge (red) and the other end has a partial positive charge (blue). The dipole-dipole force is an attractive force between the positive end of one molecule and the negative end of the neighboring molecule.
hydrogen bonds
First of all, don't let the name fool you! Although called a "bond," a hydrogen bond is not a covalent bond but a type of intermolecular force. Hydrogen bonding is the strength between an H atom bonded to O, N, or F (atoms with high electronegativity) and the neighboring electronegative atom. It can be represented generally as:
The most common example of hydrogen bonding is water molecules. Water has two O-H bonds and both are available as hydrogen bond donors for neighboring molecules. This explains the exceptionally high b.p. Water (100 °C) considering the low molar mass of 18.0 g/mol. For comparison, the methane molecule CH4with a similar size has a b.p. of -167.7 °C.
In organic compounds, hydrogen bonds play an important role in determining the properties of compounds with OH or NH bonds, e.g. alcohol (R-OH), carboxylic acid (R-COOH), amine (R-NH2) and amide RCONH2.
The three main types of intermolecular forces are summarized and compared inTable 2.6.
Polar vs. non-polar molecules
As stated inTable 2.6, the type of molecular polarity determines the type of force(s) exerted on a given substance. So here we will have discussions on how to tell if a molecule is polar or non-polar.
The polarity of the compound can be determined by its formula and form.
Fordiatomic molecules, the molecular polarity is the same as the bond polarity. This means that all homonuclear molecules like H2, N2, out of2, F2, are non-polar due to their non-polar bonding, while all heteronuclear molecules such as HF, HCl are polar.
ForMoleküle polyatomicas, the molecular polarity depends on the shape (see VSEPR onSection 1.5) of the molecule. Let's look at the examples of H2From CO2.
Both H2From CO2have two polar bonds. H2O is in folded form, so the bond polarities of the two O-H bonds add to give the molecular polarity of the entire molecule (shown above), i.e. H2O is a polar molecule. On the other hand, the form of CO2is linear and the bond polarities of the two C=O bonds cancel, hence all CO2Molecule is non-polar.
There are other examples of non-polar molecules where the polarity of the bond cancels, such as B.BF3, CCl4, PCl5, ob4etc.
For organic compounds, hydrocarbons (CXHj) are always nonpolar. This is mainly due to the small difference in electronegativity between carbon atoms and hydrogen atoms, making C-H bonds technically non-polar bonds.
Other organic compounds containing functional groups with heteroatoms such as R-O-R, C=O, OH, NH are all polar molecules.
The diagram here (Figo. 2,6 gr) provides a summary of all discussions on molecular polarities.
In addition to the three types of intermolecular forces, there is another very important interaction in understanding the physical property of a compound, and that is the ion-dipole force.
ion dipole force
The ion dipole force is not categorized as an intermolecular force, but it is an important non-covalent type of force responsible for the interaction between ions and other polar substances. A simple example is dissolving an ionic solid or salt in water. When table salt (NaCl) is dissolved in water, the interactions between the ions and the water molecules are strong enough to overcome the ionic bond that holds the ions in the crystal lattice. This completely separates the cations and anions and each ion is surrounded by a cluster of water molecules. This is calledSolutionProcedure. Solvation takes place through the strong ion dipole force. Many salts or ionic compounds are soluble in water due to such interactions.
2.6.2 Physical properties and intermolecular forces
Understanding intermolecular forces helps us to understand and explain the physical properties of substances since it is the intermolecular forces that are responsible for physical properties like phases, boiling points, melting points, viscosities, etc. For purposes of organic chemistry we will focus on boiling point (boiling point) and solubility.
Boiling point (bp):
The tendency of the boiling point of different substances directly correlates with the total intermolecular forces. Generally speaking, the stronger the general intermolecular force acts on a given substance, the higher the boiling point of the substance. 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 required to overcome the forces and a higher temperature is required, resulting in a higher boiling point.
Example:
: "2.6: Intermolecular Force and Physical Properties of Organic Compounds (10)")
All three compounds have similar molar masses here, so the dispersion forces are on a similar level. However, the three compounds have different molecular polarities. Butane is a non-polar substance that only has dispersive forces, propanal is a polar molecule with dispersive forces and dipole-dipole forces, and propanol is a polar molecule with an OH bond, so all three types of forces overlap. Therefore, the total set of intermolecular forces is strongest for propanol and weakest for butane, which is of the same order of magnitude as their boiling points.
Solubility:
A general rule of thumb for solubility can be summed up in the expression “like dissolves like”. This means that one substance can dissolve in another with similar polarity and consequently similar intermolecular forces. More accurate:
- Non-polar substances are generally soluble in non-polar solvents.
- Polar and ionic substances are generally soluble in polar solvents.
- Polar and non-polar substances are insoluble in each other.
Determining the polarity of a substance was summarized earlier in this section (Figo. 2,6 gr). Water, methanol, and ethanol are examples of very polar solvents that can form hydrogen bonds. Ethers, ketones, halides, and esters are also polar solvents, but not as polar as water or methanol. Non-polar solvents include hydrocarbons such as hexane, benzene, toluene, etc.
However, with some organic compounds, it may not be as easy to simply call them polar or non-polar, since part of the compound can be polar and part non-polar. This is often referred to as hydrophilic or hydrophobic.
- hydrophob(Water, Water;phobic: fear or avoid), meaning it dislikes water or is insoluble in water;
- hydrophil(Water, Water;phil: love or seek), meaning it likes water or is soluble in water.
The hydrocarbon partof the organic compoundhydrophob, because isnon-polar and thereforedoes not dissolve in polar water. The functional group of OH, COOH, NH2etc is polar and therefore ishydrophil. Since both hydrophobic and hydrophilic parts are present in an organic compound, the overall polarity depends on which part is the major part. When the carbon chain is short (1 to 3 carbons), the hydrophilic effect of the polar group is the main effect, so the whole compound is water soluble; at carbon chains of 4 to 5 carbons, the hydrophobic effect begins to overcome the hydrophilic effect and water solubility is lost.
Differences in the solubility of different alcohols clearly show this tendency; As the carbon chain length increases, the solubility of alcohol in water decreases dramatically (Table 2.7):
| Alcohol | solubility in water (g/100ml) |
| Methanol, Ethanol, Propanol (CH3OH, CH3CH2OH, CH3CH2CH2OH) | miscible (resolve in all shares) |
| 1-Butanol (CH3CH2CH2CH2OH) | 9 |
| 1-Pentanol (CH3CH2CH2CH2CH2OH) | 2.7 |
| 1-Octanol (CH3CH2CH2CH2CH2CH2CH2CH2OH) | 0,06 |
Table 2.7 Solubility of various alcohols in water
For organic compounds that are waterinsoluble, they can be partially converted into the "salt derivative" by a suitable reaction and thus made water soluble. This method is commonly used in laboratories to separate organic compounds.
Example:
The use of acid-base reactions is the most common way to achieve such purposes. As shown in the example above, by adding a strong base to benzoic acid, an acid-base reaction takes place and the benzoic acid is converted to its salt, sodium benzoate, which is soluble in water (due to the ion-dipole force as we saw earlier learned more). Benzoic acid can therefore be brought into the aqueous (aqueous) phase and separated from other organic compounds that do not have similar properties.