Organic Compounds and Functional Groups

There are more than 19 million known organic compounds, each with its own physical and chemical properties. This is very daunting, but organic compounds may be classified into families according to their structures. Members of the same families tend to have similar properties, so grouping compounds into families helps simplify what would initially seem to be an bewildering number of compounds.

Organic compounds are classified and grouped according to what functional groups they contain. A functional group is simply a group of atoms which is chemically reactive. In general, a functional group either contains a π bond or an electronegative or an electropositive atom(s). So the presence of π bonds or polar bonds is a good indicator of chemical reactivity.

A given functional group will undergo the same kinds of chemical reactions in most molecules in which they are present. The molecule may be very small and simple, or it may be large, complex, and bulky: but the common functional group will behave in an almost identical manner.

Common Functional Groups The following table contains many of the common functional groups which organic molecules may contain. Note that in the Table, R is used to signify an alkyl group, a group which contains only sp 3 C atoms and H atoms. When more than one R is present in a General Formula, they may or may not be different alkyl groups.

Common Functional Groups What are you responsible for? You should recognize alkenes, alkyl halides, alcohols, amines, ethers, carboxylic acids, ketones, aldehydes, esters, and amides.

Functional Groups with a C-C π Bond There are several common groups which contain a π C-C bond. What is a  bond? What is a π bond? Alkenes contain a double bond, while alkynes contain a triple bond.

Functional Groups, Cont. Arenes, where benzene is the simplest example, are 6-membered rings with alternating single and double bonds (this is a simplistic explanation, arenes have resonance structures). Although you know that double and triple bonds are shorter and stronger than single bonds, the π bond is still chemically active and can undergo certain chemical reactions.

Functional Groups with Carbon Singly Bonded to an Electronegative Atom Carbon is quite commonly bonded to an electronegative atom like O, N, Cl, Br, I, F, or S. Alkyl halides have C bonded to a halogen; ethers, alcohols have C bonded to O; amines have C bonded to N; and thiols have C bonded to S. In all of these cases, the end result is a polar bond with the C having a δ + charge. This partial charge separation leads to chemical reactivity.

Carbonyl Functional Groups There is a common class of compounds where a C atom is double bonded to O, or the C=O group. It’s generic name is the carbonyl group. Common carbonyl compounds include amino acids, fatty acids, and soaps. There are several types of carbonyl compounds as seen in the Table.

Carbonyl Functional Groups –carboxylic acids –esters (sweet smelling compounds found in fruits, etc., and used in perfumes) –ketones –aldehydes (usually foul smelling like formaldehyde) –amides (peptides and proteins are amides)

Carbonyl Functional Groups –acid halides –acid anhydrides –In all of these groups, the C=O bond is polar with the O atom having a δ - charge and the C atom having a δ + charge.

Properties of Alkanes Alkanes are hydrocarbons (C, H compounds) which contain only sp 3 hybridized C atoms. Thus, they contain only single bonds. Because of this, they are also called saturated hydrocarbons, as they are saturated with hydrogen. They have a C n H 2n+2 general formula (except cycloalkanes).

Properties of Alkanes In general, alkanes are not very reactive. They will react with oxygen, halogens, and a few other substances. Alkanes are common fuel sources for cars, houses, etc. Common alkane fuels are methane, propane, butane, and of course think of your octane rating at the gas pump!

Properties of Alkanes When an alkane (or any hydrocarbon) combusts, the two ideal products are carbon dioxide and water, although carbon monoxide and other unwanted products are made as well. Of course, combustion reactions are exothermic, so a great deal of useful heat is given off as well. This heat is called the heat of combustion, ΔH comb. As you might expect, as you increase the number of C atoms in the alkane, there is more to burn, and more heat is given off.

Alkane Halogenation Alkanes also react with halogens in a reaction called halogenation. UV light acts as a catalyst in starting the reaction. One or more halogen atoms may be added to the alkane, so there are a variety of products made.

Polarity and Intermolecular Forces Alkanes are nonpolar molecules as they contain only C-C and C-H single bonds. So the intermolecular forces acting between molecules are weak London Dispersion Forces. As you may remember, London Dispersion Forces get stronger as the molar mass or molecule size increases.

London Forces & BPt and MPt alkanes with a carbon chain up to 4 are gases at room temperature alkanes with a C chain of up to 17 are liquids alkanes with a C chain of over 17 are solids.

London Forces & Molecular Shape You may also remember, that the bulkiness or shape of the molecule also affects the strength of the London Dispersion Forces. The closer 2 or more molecules may come together, the stronger the London Forces will be, so the higher the boiling and melting point. So if a molecule is very bulky with lots of alkyl group branching, molecules can’t get as close together. Continuous-chain hydrocarbons have higher boiling and melting points than do what we call branched-chain alkanes.

Alkane Solubilities Because alkanes are nonpolar, they are not very soluble in water or other very polar solvents. Instead, they are soluble in relatively nonpolar solvents like benzene, ethers, etc. The densities of alkanes are lower than water, which you should know as oil and gas float on water!

Cycloalkanes Cycloalkanes are ring structure where there are no end C; or you could say that the two end C have joined to form a ring. Because of this extra C-C bond, 2 H atoms are lost, so the general formula is C n H 2n. But, just like alkanes, they contain only sp 3 hybridized C atoms.

Properties of Cycloalkanes The properties of cycloalkanes are very similar to those of alkanes. However, if you use a molecular model kit and make some cycloalkanes, you will notice a few differences. Cycloalkanes, particularly small cycloalkanes, are highly symmetric.

Properties of Cycloalkanes You should also notice that cycloalkanes do not rotate around C-C bonds as freely as regular alkanes. In open-chain alkanes, there is free rotation around all of the C-C bonds. But in cycloalkanes, this C-C rotation is greatly hindered by the closed ring: if there were free rotation around the C-C bond, the ring might be broken.

Properties of Cycloalkanes The smallest cycloalkanes, C 3 to C 6 or C 7, have rigid rings with little C-C rotation possible. Because of this rigidity, these rings are strained, and they are more reactive than larger rings. As cycloalkanes get larger, the rigidity of the ring decreases, the ring strain decreases, and there is more freedom of rotation. Large cycloalkanes are floppy, and are virtually identical to open-chain molecules.

Properties of Cycloalkanes Because of their greater symmetry and hindered C-C rotation, cycloalkanes generally have higher melting points, boiling points, and densities than open- chain alkanes. See the following Table for examples.

CompoundB.Pt.M.Pt.Density (g/mL) Pentane (C 5 H 12 ) Cyclopentane (C 5 H 10 ) Octane (C 8 H 18 ) Cyclooctane (C 8 H 16 )