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So, if you're interested in learning about Organic Chemistry reactions, keep reading to find out!
- This article is about Organic Chemistry reactions.
- First, we will learn about the different types of reaction mechanisms.
- Then, we will look at some important organic chemistry reactions involving Alkenes and alkynes.
- After, we will learn about reduction reaction in organic chemistry.
- Lastly, we will look at a reaction chart and some examples of organic chemistry reactions.
Organic Chemistry Reactions Mechanisms
When dealing with organic chemistry, some important topics include reaction mechanisms, reactivity, and curved arrow pushing.
In organic chemistry, curved arrows are used to show the movement of electrons as bonds are broken and formed.
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
The curve arrow basically tells you where to take a pair of electrons from (bond/lone pair), and where to move them to (bond/lone pair).
There are three types of reaction mechanisms in organic chemistry that you should be familiar with:
Nucleophilic substitution bimolecular Reaction Mechanism (SN2)
Elimination, bimolecular reaction mechanism (E2)
Nucleophilic substitution unimolecular reaction mechanism (SN1)
SN2 Reaction Mechanism
Nucleophilic substitution reactions are those in which the swapping of one functional group from another occurs. The SN2 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-.
A nucleophile is an electron pair donor. Most strong nucleophiles have a negative charge.
SN2 reactions are considered bimolecular, meaning that two reactant molecules are involved in the slow-step. In these reactions, the substrates are often alkyl halides.
SN1 Reaction Mechanism
Compared to SN2 reactions, SN1 reactions are unimolecular, so only one reactant is involved in the slow step of the reaction. The intermediate in SN1 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.
An alkene is a hydrocarbon with at least one C—C double bond. For example, propene is an alkene.
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.
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.
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.
These reactions are only a small part of all the different reactions involved in organic chemistry, and you will learn more about them in your organic chemistry university course!
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.
By using the reagents lithium aluminum hydride (LiAlH4) and H2O to reduce Esters into alcohols.
Using lithium aluminum hydride (LiAlH4) and an acid as reagents to reduce Carboxylic Acids into alcohols.
Using sodium borohydride (NaBH4) or LiAlH4 to convert aldehydes into alcohols.
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.
Knowing your reagents is very important in organic chemistry because by knowing how the different reagents work, you can predict the products that will form in different organic chemistry reactions!
Let's look at an example.
What would be the product of the following organic chemistry reaction?
When you see a problem like this, the first thing you need to do is the type of reactant and reagent that you have. In this case, we have a ketone, and we are adding NaBH4 to it. This is an example of reduction of ketone, which results in the formation of a secondary alcohol.
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.
In a reaction involving an aldehyde, the addition of a Grignard reagent (and an acid in the second step) converts the aldehyde into a secondary alcohol.
Now, I hope that you were able to understand organic chemistry reactions a little better!
Organic Chemistry Reactions - Key takeaways
- In organic chemistry, curved arrows are used to show the movement of electrons as bonds are broken and formed.
- Organic chemistry reactions involving alkenes include hydrohalogenation, acid-catalyzed hydration, dihydroxylation, and oxymercuration.
- Important organic chemistry reactions of alkynes include limination of hydrogen halides, hydrogenation, and chlorination.
References
- Zumdahl, S. S., Zumdahl, S. A., & Decoste, D. J. (2019). Chemistry. Cengage Learning Asia Pte Ltd.
- Solomons, G., Fryhle, C. B., & Snyder, S. A. (2016). Organic chemistry II. John Wiley & Sons, Inc.
- Master Organic Chemistry - An Online Organic Chemistry Resource. (2019). Master Organic Chemistry. https://www.masterorganicchemistry.com/
- Chad’s Videos. (n.d.). Chad’s Prep -- DAT, MCAT, OAT & Science Prep. Retrieved September 28, 2022, from https://courses.chadsprep.com/courses/take/organic-chemistry-1-and-2
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Frequently Asked Questions about Organic Chemistry Reactions
What are the five main types of organic reactions?
Types of organic reactions include addition, elimination, and substitution reactions.
What is an organic chemistry reaction?
An organic chemistry reaction is a reaction that involves carbon-containing compounds.
How many reactions are in organic chemistry?
In organic chemistry, there are more than 100 different reactions.
Why are organic reactions important?
Organic reactions are important because they show how carbon-containing compounds react
What is the easiest way to remember organic chemistry reactions?
The easiest way to remember organic chemistry reactions is to make an organic chemistry chart or map of the reactions.
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