HS Chemistry - Introduction to Organic Chemistry

Organic Compound Reactions

Overview of The Page

This page will cover:

• What basic types of reactions do organic compounds undergo?
• What basic reaction mechanisms occur in those reactions?

Organic compounds often react with each other in nature. The different types of reactions that occur, as well as their different reaction mechanisms, are important to know. There are other types of reactions that occur with specific functional groups, but these are the main types of reactions. Before we start defining them, however, we need to understand what electrophiles and nucleophiles are.

Electrophiles are atoms/ions/molecules that attract, or sometimes "steal", electrons in reactions. Electrophiles often have a deficit of electrons, and therefore draw electrons in reactions, initiating the reaction. They often break double or triple bonds between Carbon atoms, or substitute out less electronegative atoms.

Nucleophiles are atoms/ions/molecules that donate, or "give away", electrons in reactions. Nucleophiles often have excess electrons that initiate the reaction. They are often present in addition and substitution reactions.

Types of Reactions

• Addition reactions involve two molecules reacting and combining into one larger molecule. This typically consists of both an electrophilic and a nucleophilic addition, although sometimes only one occurs. A common example of this is when an alkene reacts with a halogen to form a halogenoalkane $covered more in [Alkenes](../Unit-10/3-Alkenes.md)$.

• Substitution reactions involve one atom/ion/molecule replacing another atom/ion/molecule's place in a larger organic molecule. This can be carried out by both electrophiles and nucleophiles, and also by free radicals $covered later in the page$. A common example of this is when a halogen atom substitutes with a Hydrogen atom in an alkane to form a halogenoalkane $covered more in [Alkanes](../Unit-10/2-Alkanes.md)$.

• Elimination reactions are essentially the reverse of addition reactions - one molecule breaks up into two or more smaller molecules. Sometimes, another molecule will facilitate this process, causing it to occur. This, like addition, typically consists of both an electrophilic and a nucleophilic part, although sometimes only one occurs.

• Hydrolysis reactions occur when water $or a dilute acid or base$ breaks down a larger organic molecule into two or more smaller molecules. This typically consists of a nucleophilic substitution, where a hydroxide $OH^\-^$ ion $which is a nucleophile, as it has an extra electron that it can donate$ will substitute with a part of the larger molecule.

• Condensation reactions occur when two organic molecules react and combine together, and in the process, a small molecule $such as water$ is removed. This typically consists of both an electrophilic and a nucleophilic part.

• Originally, oxidation reactions referred to the addition of Oxygen atoms to an organic molecule, which is where it gained its name from. Its definition was later expanded to include the loss of electrons from an atom/ion/molecule. However, in organic reactions, oxidation is more commonly used to refer to a gain in the number of bonds between Carbon and a more electronegative atom $usually Oxygen$ OR a reduction in the number of bonds between Carbon and a less electronegative atom $usually Hydrogen$.

  This typically occurs when one of the reactants has been acidified, and its ions have been displaced in solution. To show this, Oxygen atoms are often written as [O] in the equation. The equation must still be balanced, however, treating each [O] as one Oxygen atom.

  In an oxidation reaction, the compound that gives the [O] is referred to as the oxidizing agent. Again, though, it can be any atom that's more electronegative than Carbon. This means that halogen additions are technically oxidation reactions as well. However, it is more descriptive to say that a halogen addition reaction is occurring.

  Some examples of commonly used oxidizing agents are:

  1. Acidified Potassium Dichromate $K~2~Cr~2~O~7~ dissolved in H~2~SO~4~$: It will oxidize primary alcohols to aldehydes, aldehydes to carboxylic acids, and secondary alcohols to ketones. It will change color from orange to green over the course of the oxidation reaction.

  2. Acidified Potassium Permanganate $KMnO~4~ dissolved in H~2~SO~4~$: It will oxidize primary alcohols to aldehydes, aldehydes to carboxylic acids, secondary alcohols to ketones, and alkenes to diols $molecules with two alcohol functional groups$. Same function as acidified potassium dichromate, except that it can also oxidize alkenes to diols, and has a different color. It will lose its purple color over the course of the oxidation reaction.

• Originally, reduction reactions referred to the decrease, or reduction, of Oxygen atoms from an organic molecule, which is where it gained its name from. Its definition was later expanded to include the gain of electrons to an atom/ion/molecule. However, in organic reactions, reduction is more commonly used to refer to a reduction in the number of bonds between Carbon and a more electronegative atom $usually Oxygen$ OR a gain in the number of bonds between Carbon and a less electronegative atom $usually Hydrogen$.

  This, like oxidation, typically occurs when one of the reactants has been acidified, and its ions have been displaced in solution. To show this, Hydrogen atoms are often written as [H] in the equation. The equation must still be balanced, however, treating each [H] as one Hydrogen atom.

  In a reduction reaction, the compound that gives the [H] is referred to as the reducing agent. Again, though, it can be any atom that's less electronegative than Carbon. This means that halogen eliminations are technically reduction reactions as well. However, it is more descriptive to say that a halogen has been removed from the molecule.

  Some examples of commonly used reducing agents are:

  1. Sodium Borohydride $NaBH~4~$: It will reduce carboxylic acids to aldehydes, aldehydes to primary alcohols, and ketones to secondary alcohols.

  2. Lithium Aluminum Hydride $LiAlH~4~$: It will reduce carboxylic acids to aldehydes, aldehydes to primary alcohols, and ketones to secondary alcohols. Same function as sodium borohydride.

In addition to the types of reactions, the ways in which these reactions occur $called the reaction mechanisms$ help determine what the effects of the reaction will be.

Reaction Mechanisms - Homolytic Fission

When organic molecules react, they first have to break apart bonds before they can form new ones. When single bonds break apart, either each atom can get one electron, or the more electronegative atom in the bond can take both electrons. Homolytic fission is when each of the two atoms gets one of the electrons that were shared in the bond. As a result of this, each now has 7 valence electrons, and wants to react with other atoms to gain one more electron and fill its valence shell. This is called a free radical, and it is very reactive. Free radicals have a dot next to the atom's name when writing the equation to represent the unpaired electron.

Free radicals can initiate their own type of reaction, called a free-radical reaction. A free-radical reaction consists of three parts:

1. Initiation: Homolytic fission occurs, splitting the molecule and forming two free radicals.

2. Propagation: A free radical reacts with, or attacks, an atom/molecule that isn't a free radical. It steals an electron from that atom/molecule to complete its valence shell, and no longer remains a free radical. However, the atom/molecule from which the electron was stolen is now a free radical. The number of free radicals remains unchanged.

3. Termination: Two free radicals react with each other and form a single molecule. Both free radicals are now gone, and there are no free radicals left in their wake.

Homolytic fission tends to happen when a non-polar molecule is broken, or when the difference between the electronegativities of the two atoms isn't too large.

Finally, homolytic fission is often shown in a chemical reaction using the following symbol:

Where the half-arrows point towards both atoms between which the bond is being broken.

Reaction Mechanisms - Heterolytic Fission

When organic molecules react, they first have to break apart bonds before they can form new ones. When single bonds break apart, either each atom can get one electron, or the more electronegative atom in the bond can take both electrons. Heterolytic fission is when the more electronegative atom takes both of the electrons that were shared in the bond. This leaves the other side as a positively charged ion.

Often times, heterolytic fission in an organic molecule will occur with an alkyl group in the organic molecule, leaving the Carbon atom with a positive charge. This positively charged alkyl group, which is missing an electron, is called a carbocation. There are three types of carbocations:

1. Primary carbocations are formed when the Carbon atom that is left with the positive charge is bonded to two other Hydrogen atoms and one other group. Alkyl groups tend to slightly repel electrons away from them and towards the next alkyl group. They are said to have a positive inductive effect on the Carbon atom. Since primary carbocations are only attached to one alkyl group, there aren't enough electrons that are being pushed by the alkyl group slightly towards the positively charged Carbon ion. This causes the positively charged Carbon ion to have a strong charge, which makes it more reactive, and therefore less stable than secondary and tertiary carbocations, which makes it the least stable type of carbocation.

2. Secondary carbocations are formed when the Carbon atom that is left with the positive charge is bonded to one other Hydrogen atom and two other groups. Since the surrounding alkyl groups have a positive inductive effect on the positively charged Carbon ion, the presence of more alkyl groups will create a greater positive inductive effect. Thus, in secondary carbocations, the Carbon ion has a weaker charge than in primary carbocations (although still stronger than in tertiary carbocations). Therefore, secondary carbocations are more stable than primary carbocations but less stable than tertiary carbocations.

3. Tertiary carbocations are formed when the Carbon atom that is left with the positive charge isn't bonded to any Hydrogen atoms, but is instead bonded to three other groups. Since the surrounding alkyl groups have a positive inductive effect on the positively charged Carbon ion, the presence of more alkyl groups will create a greater positive inductive effect. Thus, in tertiary carbocations, the Carbon ion has a weaker charge than in primary or secondary carbocations. Therefore, tertiary carbocations are more stable than primary and secondary carbocations, which makes them the most stable type of carbocation.

After being formed, the carbocation will react with another atom/molecule to get rid of its positive charge. As a result, carbocations are mostly seen in the middle of a reaction, not the beginning or end. However, they can help us determine the product molecules.

When reactions occur, organic compounds are most likely to assume the most stable state, even in the middle of a reaction. This means that in an organic reaction, the most stable carbocation possible for that reaction is the one most likely to form. Thus, tertiary carbocations, where possible, are more likely to form than secondary carbocations, and secondary carbocations, where possible, are more likely to form than primary carbocations in reactions.

For this reason, carbocations will tend to form on Carbon atoms that aren't at the edges of an alkane if they can. This helps explain why in substitution reactions with larger molecules, it is common to see the substituting atom take its new place near the "middle" of the organic molecule.

Finally, heterolytic fission is shown with the arrow pointing to the atom that gets both electrons, not just one.