Polarity (chemistry)
In chemistry, polarity denotes the formation of separate charge centers in atomic groups due to charge shifts; as a result, the atom groups are no longer electrically neutral.
The electric dipole moment is a measure of the polarity of a molecule and determines the solubility of a substance or its ability to act as a solvent :
- Polar substances are usually readily soluble in polar solvents, but poorly in non-polar solvents.
- Conversely, non-polar substances are soluble in non-polar solvents (e.g. in gasoline or hexane ), but poorly soluble in polar solvents .
One of the tenets of medieval alchemy was: "Similia similibus solvuntur" (Latin: "Similar dissolves in similar").
Due to their ionic structure , many salts are readily soluble in the polar solvent water , but non-polar substances such as fats or waxes are not. Many aromas and fragrances are also not soluble in water, but are readily soluble in oil or ethanol . Alcohol is therefore listed as an ingredient in many low-fat foods.
Polar fabrics
Polar substances consist of polar molecules, which are characterized by a permanent electrical dipole moment.
Polar substances dissolve well in polar solvents - as is the case, for example, with salts in water. The more similar the interaction forces between the particles of the solvent and those of the solute, the better the solubility.
If the difference in electronegativity (ΔEN) is sufficiently high , the binding electrons can almost completely pass from one binding partner to the other. Two ions remain , which only attract each other due to the undirected electrostatic Coulomb force . Ions, and thus also all salts, are fundamentally polar as charge carriers.
The polarity of an entire molecule is caused by polar atomic bonds , or in extreme cases by ionic bonds . Polar bonds are characterized by the uneven distribution of binding electrons between the binding partners. If atoms with different electronegativity connect , this can result in such a polarization of the bond. If there are only polarized atomic bonds in a molecule, the individual dipole moments of the bonds add vectorially to form a total dipole moment. If this is zero due to symmetry, the substance is non-polar (example: carbon dioxide, CO 2 ). However, if there is a permanent total dipole moment other than zero, the molecule is polar (example: water molecule ). Depending on the size of this total dipole moment, a substance is more or less polar. The difference therefore goes from extremely polar to completely non-polar. Solvents are arranged in an elutropic series based on their polarity .
In organic chemistry, polar atomic bonds play an important role in the qualitative assessment of the reactivity of a molecule. In a haloalkane (example: chloromethane) z. B. assigned the partial charge δ− to the chlorine atom covalently bonded to the carbon atom and the partial charge δ + to the carbon atom of the methyl group. Substituting chloromethane with magnesium to give the corresponding Grignard compound CH 3 MgCl to, occurs Umpolung a: The carbon atom of the methyl group now has the partial charge δ-. Considering the polarity of organic substances has significant consequences for their reactivity.
Polar substances: water , salts , sugar , glass
Non-polar substances
A non-polar or non-polar molecule, on the other hand, has no permanent dipole moment.
Non-polar substances dissolve well in non-polar solvents (organic substances in benzene or ether ). The more similar the interaction forces between the particles of the solvent and those of the solute, the better the solubility.
Non-polar substances: gasoline , carbon tetrachloride , wax , grease , alkanes , alkenes , alkynes
Determination of polarity
Experiment to prove the permanent electrical dipole moment of water
One loads z. B. Electrically open a plastic comb by combing dry hair or rubbing a wool sweater. Now you let a very thin stream flow from a tap, just so that it does not tear off and drip. If the ridge is approached to the water jet, it is deflected towards the ridge. (When the water jet touches the comb, it is discharged and no longer attracts the water jet.)
In the concentric electric field that surrounds the charged comb, the dipoles of the water molecules align themselves so that they point towards the comb. Since the field strength decreases with distance from the comb, a somewhat greater attractive force acts on the end of the molecule that is closer to the comb than the repulsive force that acts on the end of the molecule further away. A small force remains in the difference, which attracts the water molecules and deflects the water jet.
Electronegativity
To determine whether a compound is non-polar, polar or even an ionic bond, one can use the electronegativity difference . It is the difference in the electronegativity values of the atoms involved. Guide values for this classification can be seen in the table below.
However, it must be taken into account that charge-separated mesomeric limit formulas can have a weight that cannot be neglected. Despite an electronegativity difference of about 1 , carbon monoxide is an almost non-polar gas that can only be liquefied by pressure below −140 ° C.
| Binding type | Characteristics of the bond | |
|---|---|---|
| 0.0 | non-polar bond | Electron pairs are stressed equally by all atoms, so that no centers of charge arise. |
| 0.1 ... 0.4 | weak polar bond | One atom puts a slightly stronger strain on electron pairs than the other. |
| 0.4 ... 1.7 | strongly polar bond | One atom puts a much greater strain on electron pairs than the other. |
| > 1.7 | Ionic bond | There are no electron pairs in common; That is, ions are formed |