Lesson: Chapter - 10

Organic Chemistry

Simple Organic Compounds

Organic chemistry is the branch of chemistry that studies carbon compounds. This field is very important since carbon compounds are all around us—they make up a wide array of common substances such as plastics, oil, gasoline, and alcohols and are also a part of many of the foods we eat, such as proteins, carbohydrates, and fats.

One reason that there are so many carbon compounds is because of carbon’s unique ability for bonding. Recall the electron structure of carbon:

Carbon has four valence electrons, and these electrons can hybridize into sp³, sp², and sp atomic orbitals. This enables carbon to join with other elements and be involved in single, double, and triple bonds. Organic compounds that contain only carbon and hydrogen are called hydrocarbons.

Some general properties of organic compounds are

1. They usually have low melting points.
2. They usually are nonpolar (unless they bear functional groups).
3. They are usually nonconductors of electricity.
4. They can exist in solid, liquid, and gaseous form. Compounds with:

• 1–4 carbons tend to be gases at room temperature; butane and propane are among the lightest hydrocarbons and are used for fuel
• 5–10 carbons tend to be in the liquid state at room temperature; compounds that fall in this size range are used to make gasoline and solvents
• 12–18 carbons make up jet fuels and kerosene
• More than 18 carbons tend to be solids at room temperature

Organic compounds can exist as polymers, in which many repeating units (called monomers) make up a larger molecule. Amino acids are monomers of proteins when amino acids are bonded in a chain, they make a polypeptide or protein. Starches are polymers of the monomer glucose. Plastics are polymers of organic molecules extracted from crude oil. Some common examples include

• Polyethylene—Many ethenes strung together with covalent bonds (ethylene is another name for ethene); shopping bags and plastic bottles are made of polyethylene.
• Polypropylene—Many propenes strung together; glues and carpets are made of polypropylene.
• Polystyrene—A clear, hard, brittle polymer used in CD cases; if you blow carbon dioxide into it during manufacture and you get the soft, opaque, foamy polymer used in a coffee cup.

Common Functional Groups

Functional groups are atoms or groups of atoms attached to an organic compound that impart characteristic shapes and chemical properties to the compound. There are a few functional groups that you should be able to recognize for the SAT II Chemistry test.

1. Hydroxyl group, —OH: Compounds that contain an —OH group are considered alcohols. An example of an alcohol, ethanol, is shown below:
2. Carboxylic acid group, —COOH: In a carboxyl group, the carbon is doubly bonded to one oxygen and singly bonded to an OH group. An example of an organic compound containing a carboxyl group, trichloroethanoic acid, is shown below:
3. Amine group, –NH₂: An amino group contains a nitrogen and two hydrogens. In organic chemistry, an R, like the one in the diagram below, is often used as a shorthand notation to signify the rest of the molecule. This notation is generally used when only a specific part of the molecule is being discussed.

Naming Organic Compounds

Take this opportunity to look through Appendix II, Chemical Formulas Review, if you need a refresher on how to name organic compounds. It might come in handy on test day.

Isomerism

Isomers are compounds that have the same number and kinds of atoms but have different structures—meaning that the atoms are arranged differently in the molecule. Generally the number of isomers increases dramatically as the number of carbon atoms increases because there are more options for molecular structure.

1. Different carbon skeletons (one or more bonds differ). An example is C₄H₁₀:
2. Different functional groups. An example is C₂H₆O:
3. Different positions of functional groups. An example is C₃H₇NH₂.

Don’t worry about memorizing all of the names of these compounds. They are only included to show that isomers are completely different compounds, with different names.

You will be expected to be familiar with a few simple, common organic reactions for the SAT II Chemistry exam, so we’ll go through them briefly now. Let’s start with combustion reactions. Combustion reactions are reactions that occur between oxygen and hydrocarbons, or C_xH_yO_z. There are two main types of combustion reactions—complete and incomplete.

Complete combustion occurs when excess oxygen is present; this type of reaction produces carbon dioxide and water.

CH₄ + 2O₂ ? CO₂+ 2H₂O

Incomplete combustion occurs when a limited amount of oxygen is present, and the products of incomplete combustion are often difficult to determine. There may be carbon monoxide, carbon, and water or some mixture of all of these. When cooking outdoors on a grill, you often are left with pure carbon (soot) on utensils. Space heaters and automobiles often undergo incomplete combustion and produce deadly carbon monoxide (CO) gas. Here’s the reaction for an incomplete combustion:

2CH₄ + 3O₂ ?2CO+ 4H₂O

Another common organic reaction is called an addition reaction. In an addition reaction, two reactants join to form a single product:

Finally, we have the substitution reactions. In a substitution reaction, one group replaces another group on the main carbon chain. The atom that’s most commonly replaced in a substitution reaction is hydrogen. One common example of this is halogenation, which is the addition of a halide—remember, group 7A on the periodic table.

CH₄ + Cl₂ ? CH₃Cl + HCl

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