Organic Chemistry Reactions

Organic Chemistry Reactions Mechanisms

When dealing with organic chemistry, some important topics include reaction mechanisms, reactivity, and curved arrow pushing.

For example, to show the movement of electrons in a chemical reaction involving OH and HCl, the Reaction Mechanism would look like this: First, a bond will form between oxygen and hydrogen. Then, the bond between H and Cl will break, and the pair of electrons in the bond will become a new lone pair on Cl.

_{Fig.1-Curved arrow pushing in reaction between OH and HCl}

There are three types of reaction mechanisms in organic chemistry that you should be familiar with:

1. Nucleophilic substitution bimolecular Reaction Mechanism (S_N2)

2. Elimination, bimolecular reaction mechanism (E2)

3. Nucleophilic substitution unimolecular reaction mechanism (S_N1)

S_N2 Reaction Mechanism

Nucleophilic substitution reactions are those in which the swapping of one functional group from another occurs. The S_N2 mechanism proceeds through a backside attack (a nucleophile attacks the back of the carbon-leaving group.

• Good leaving groups tend to be weak bases such as I^-, Br^-, Cl^-, H₂O, AcO^-, TsO^-.

• Good nucleophiles include OH-, CH₃O^-, H₂N^-, N₃^-, and HS^-.

S_N2 reactions are considered bimolecular, meaning that two reactant molecules are involved in the slow-step. In these reactions, the substrates are often alkyl halides.

S_N1 Reaction Mechanism

Compared to S_N2 reactions, S_N1 reactions are unimolecular, so only one reactant is involved in the slow step of the reaction. The intermediate in S_N1 reactions is called a carbocation.

E2 Reaction Mechanism

During Elimination Reactions, the loss of a leaving group from the α-carbon and a hydrogen from the β-carbon happens, resulting in the formation of a double bond. Elimination Reactions require a strong base and is generally favored with heat.

• Good bases include HO^-, CH₃O^-, H₂N^-, RO^-, R₂N^-, H^-, and R^-.

Important Organic Chemistry Reactions

Now that you know how the movement of electrons are shown in different reaction mechanisms, let's take a look at some important organic chemistry reactions.

Reactions of Alkenes

Let's start with some reactions involving Alkenes. These are hydrohalogenation, acid-catalyzed hydration, dihydroxylation, and oxymercuration.

In the hydrohalogenation of alkenes, hydrobromic acid (HBr), hydrochloric acid (HCl) or HI can be used to convert alkenes to alkyl halides. The image below shows the addition of H—Br to an alkene. During this mechanism, the alkene attacks the acid (HBr) and forms a carbocation. Then, the nucleophile (Br- in this case) attacks the carbocation, forming an alkyl halide.

• An alkyl halide is an alkyl group (a hydrocarbon group with no multiple bonds) attached to a halogen.

In an acid-catalyzed hydration reaction, reagents such as aqueous acid (H₃O⁺) or H₂O/H₂SO₄ are used to give an alcohol as a product.

Fig.6-Hydration of 1-butene

Next, we have dihydroxylation of alkenes to form vicinal diols. In other words, the double bond is broken and replaced by 1,2-diols. This reaction occurs under cold, dilute basic conditions using KMnO₄ as a reagent.

Another important organic chemistry reaction of alkenes is oxymercuration. In this reaction, the end product depends on the reagents used.

• To convert alkenes to Alcohols, we first add Hg(OAc)₂, H₂O and then NaBH₄. The NABH₄ step removes the mercury.

• To convert alkenes to ethers, we use Hg(OAc)₂, HOCH₃, and then NaBH₄. An ether is just an oxygen atom flanked by two carbons. Dimethyl ether (H₃C—O—CH₃) is a common example of an ether.

Fig.8-Oxymercuration of alkenes

Reactions of Alkynes

Alkynes are hydrocarbons with at least one C—C triple bond. Although there are many reactions involved with alkynes, let's take a look at the elimination of hydrogen halides, hydrogenation, and Chlorination.

During the elimination of hydrogen halides, sodium amide (NaNH₂) is used as a strong base reagent to eliminate HBr and form an alkyne.

Alkyne hydrogenation is a reaction that converts alkynes into alkanes by using hydrogenation catalysts such as Pd/c, Pt, and Ni.

• Hydrogenation can also be used to convert alkenes into alkanes.

Now, alkyne Chlorination, as the same suggests, is the addition of chlorine (Cl2) to an alkyne, turning it into an alkene, and then to an alkane.

Reduction Reactions in Organic Chemistry

Reduction reactions also play a big role in organic chemistry and are seen in alcohol synthesis reactions, alkyne reactions, and also in reactions containing other Functional Groups.

• An aldehyde is a carbonyl (C=O) attached to a hydrogen and another Carbon.

• A ketone is a carbonyl (C=O) flanked by two carbons.

• A carboxylic acid is a carbonyl (C=O) adjacent to a hydroxyl (OH) and an R group.

Let's start with reduction reactions involving Alcohols. There are four important ways to form alcohols through reduction.

1. By using the reagents lithium aluminum hydride (LiAlH₄) and H₂O to reduce Esters into alcohols.

2. Using lithium aluminum hydride (LiAlH₄) and an acid as reagents to reduce Carboxylic Acids into alcohols.

3. Using sodium borohydride (NaBH₄) or LiAlH₄ to convert aldehydes into alcohols.

4. Using NaBH₄ to reduce ketones into alcohols.

The image below shows examples of these reduction reactions that lead to the formation of an alcohol:

Reduction reactions are also seen in alkynes. In this case, Na/NH₃ is used as a reagent to reduce an alkyne into an alkene.

A common reagent that can be used in reduction reactions is DIBAL. DIBAL is a strong, bulky reducing agent that is greatly used in the reduction of Esters into aldehydes, ketones into secondary alcohols, aldehydes into primary alcohols, and Nitriles to aldehydes.

Let's look at an example.

Organic Chemistry Reactions Chart

Since there are so many types of reactions, reagents and Functional Groups, chemists use a reaction chart (or reaction map) to help them figure out what is going on in an organic chemistry reaction.

The figure below shows an organic chemistry reaction chart for reactions of alkanes, alkyl halides, alkenes, alkynes, and alcohols.

Organic Chemistry Reactions Examples

Now that we know about important organic chemistry reactions, let's dive into another interesting example of organic chemistry reactions!

Grignard reagents are a vital part of organic chemistry. They can be used to convert many functional groups into alcohols. Grignard reagents are considered good nucleophiles and very strong bases.

Now, I hope that you were able to understand organic chemistry reactions a little better!