Organic Reactions

Now that I've still got some time before the exam, I'm going to go over some of the basics that we learned at the start of the semester.

Determine the type of reaction based on reactants and products.

This dot point is kind of vague, because there are many different ways of classifying reactions. The first few slides talk about addition, elimination, substitution and rearrangement reactions, so I'm going to go from there.

Addition reactions occur when you have a double bond which breaks to add other stuff to the molecule. For example, ethene and Br2 can undergo an addition reaction to form 1,2-dibromoethane. Note that double bonds are required for such reactions to occur. This is why compounds with double bonds (especially those that do not have resonance) are generally more reactive than their fully-saturated counterparts.

Elimination reactions are essentially the opposite of addition reactions. Atoms are released, leaving a double bond behind.

Substitution reactions occur when one element is substituted for another. For example, ethane and Br2 can react under UV light to form bromoethane and hydrogen bromide.

Rearrangement reactions are reactions in which functional groups move to a different place in the molecule.

I've also got a bit more information about addition and substitution reactions on an earlier post about the basics of organic chemistry.

Draw simple organic reaction mechanisms, such as radical and polar reactions.

Sorry, but I'm too lazy to draw right now so I'm just going to provide a written explanation until someone nags me for a diagram.

Generally two electrons are involved in a bond between two atoms. If one of these electrons is provided by one atom and the other electron is provided by the other atom, then the bond-making process is said to be symmetrical, or radical. If, when the bond breaks, one electron goes to one atom and the other goes to the other atom, this bond-breaking process is also said to be symmetric, or radical. Radicals are atoms with at least one unpaired electron in their outer shell, and they are very reactive. They can be neutralised by other molecules called antioxidants, though they're not important to learn about right now.

Asymmetrical bond-making and bond-breaking processes, in which both electrons come from the same atom when forming or go to the same atom when breaking, are also possible. These reactions are called polar reactions.

Identify a nucleophile and an electrophile.
Identify the nucleophilic and electrophilic parts of the molecule.

Nucleophiles, or "nucleus-lovers," are atoms that have at least a slight negative charge and are thus drawn to atoms with a slight positive charge. Electrophiles are the opposite. Negatively-charged particles, such as Cl- ions are always nucleophiles, and positively-charged particles, such as Na+ ions, are always electrophiles. Neutrally-charged molecules can also have nucleophilic and electrophilic regions: for example, the O atom of water tends to attract electrons more strongly than the H atoms, and so the O atom has a slight negative charge and is nucleophilic, whereas the H atoms have slight positive charges and are electrophilic. Note that neutrally-charged molecules that have nucleophilic sites must also have electrophilic sites, and vice versa, in order to balance out the charges across the molecule.

Identify the four general reactions involving carbonyl compounds.
Describe the mechanism of Nucleophilic Acyl Substitution Reactions.

There are four main reactions involving carbonyl compounds: nucleophilic addition reactions, nucleophilic acyl substitution reactions, alpha substitution reactions and carbonyl condensation reactions. (I don't think we need to know much about the latter two types, but I'll include them for completeness.) This is another section that would be better with pictures, but I'm lazy.

In a nucleophilic addition reaction, a nucleophile "attacks" a carbonyl carbon. As electrons from the nucleophile are transferred to the carbon, electrons are passed on to the carbonyl oxygen, breaking the double bond and changing the carbonyl oxygen to O-. This intermediate state is known as an alkoxide ion. The O- can then be protonated to form an -OH group. Sometimes this -OH group can be later eliminated as an OH- ion or water to give a product with a double bond between the carbonyl carbon and the attacking nucleophile.

In an nucleophilic acyl substitution reaction, a nucleophile attacks a carbonyl carbon that is also singly bonded to a good leaving group, like Cl. (Good leaving groups are those that are stable on their own in solution. Highly polarised groups are also good leaving groups as they make the carbonyl carbon more electrophilic.) Nucleophilic acyl substitutions start off with an addition step similar to a nucleophilic addition reaction. However, rather than the O- being protonated to form -OH, the C=O bond is restored, transferring electrons to the leaving group in the process. The leaving group then breaks off. Note that ketones and aldehydes do not undergo nucleophilic acyl substitution as they do not have good leaving groups. (Additionally, the fact that the carbonyl carbon in ketones is "buried" in the molecule may hinder the addition step.)

Alpha-substitution reactions occur at the carbon next to the carbonyl carbon. In acidic conditions, the electrons making up the C=O double bond can move, forming a C=C double bond and O- instead, otherwise known as an enolate ion. Addition of a nucleophile can then occur at the alpha carbon, moving the double bond back to its original C=O location.

Carbonyl condensation reactions are, from what I can tell, like alpha-substitution reactions, but the nucleophile to be added is simply another carbonyl compound. Mind you, I'm not 100% sure on this one.

Describe the differences between reactions in the laboratory and biological reactions.

Reactions in the laboratory generally have a limited number of reactants under controlled conditions. Biological reactions, however, take place in "busier" environments with many reactions occurring. Also, biological systems have plenty of enzymes that catalyse specific reactions, whereas catalysts used in the laboratory tend to be more general.