Bonding Polarity and Functional Groups Help
In covalent bonds, when electrons are shared by the same elements, the electron distribution is symmetrical. If the atoms of two or more different elements share a pair of electrons, the electron density at one or another of the atoms will be different. Some atoms have a stronger attraction for a shared electron pair than another. The bonding is not equal in the general sharing (one atom gets more time with the electron pair than the other). This unequal sharing causes a slight charge shift to one atom more than the other.
Bond polarity occurs when electron pairs are unequally shared between atoms of different elements.
It is important to keep in mind that some molecules may have polar bonds, without molecular polarity, due to the canceling effects of symmetry. In carbon tetrachloride (CCl 4 ) there are four polar bonds, but the bond angles and symmetry causes the bond dipoles to cancel out. Carbon tetrachloride, then, is a non-polar molecule.
This unequal sharing or stronger/weaker attraction of shared electrons explains why some molecules have certain reactivity with some atoms and very different reactivity with others. Melting and boiling points as well as the ability to form higher energy compounds depends on these polarization effects.
In organic chemistry, carbon combines with oxygen and hydrogen in specific ways to produce standard functional groups such as alcohols, phenols, and carboxylic acids (−OH); aldehydes, ketones, esters, and carboxylic acids (C=O); amines (N−H); and nitriles (C≡N), which then react in known ways.
Some of these organic molecules are long and complex. To save time and space, certain functional groups or specific groupings of elements that do a certain job are written in chemical shorthand.
To highlight added functional groups, R is commonly used to stand for the carbon group CH 3 −, CH 3 CH 2 −, CH 3 CH 2 CH 2 −, or CH 3 CHCH 3 −.
The following shorthand formulas show how a few functional groups are added to the base carbon group (R):
- Ethanol is CH 3 CH 2 OH or R−OH
- Acetic acid (carboxylic acid, commonly known as vinegar) is CH 3 CO 2 H or R−COOH
- Formaldehyde (methanal) is R−HC=O
- Methylamine is CH 3 NH 2 or R−NH 2
- Benzylchloride is a benzene ring−CH 2 Cl.
Figure 10.7 shows some functional groups that combine with carbon to make organic molecules.
These functional groups make deciphering organic reactions much simpler. Finding functional groups on a long carbon chain (R) makes identifying the core molecule easier. Once the central compound is known, the other reactants can be recognized.
Practice problems for these concepts can be found at – Organic Chemistry and Functional Groups Practice Test
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