Polar Reaction Mechanisms: Types & Examples

Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

In this lesson, we will discuss polar reaction mechanisms of organic molecules. These mechanisms help us interpret how and why we observe particular products from a chemical reaction.

Introduction to Polar Mechanisms

We're constantly learning how to make better products: everything from improved plastics to weird new food items to better batteries for our electric cars. We can learn quite a bit by simply experimenting with different materials. In fact, this is how the tungsten filament for the light bulb was discovered!

One of Edison
edisons early lightbulb

Chemists often approach the problem by learning more about how a molecule is transformed into a completely different one. How the chemical reactants transform to products is called the reaction mechanism. In this lesson, we'll explore four types of reaction mechanisms commonly encountered among organic molecules (those containing carbon). The common denominator they share is that negative ions and (partially) negatively charged atoms--called nucleophiles--always 'attack' positively charged ions or atoms--called electrophiles.

SN1 Mechanism

SN1 stands for 'nucleophilic substitution, 1st order.' When we measure the rate at which a chemical reaction occurs, we want to know what variables can affect this rate. We have two main chemical species to consider: the nucleophile (it 'loves' positive charges) and the electrophile, which we call the substrate. For an SN1 reaction, the identity and concentration of the substrate mainly affects the reaction rate (as do other factors), but the nucleophile doesn't.

So, why does the nucleophile not affect the reaction rate? Let's consider the following example. Adding hydroxide, OH-: to 3-iodo-1-propene forms allyl alcohol:

SN1 mechanism

We can see two steps in the reaction: (1) I- leaves the substrate, forming a positively charged carbocation and (2), OH- attacks the carbon atom where the positive charge is located.

It takes very little time for OH- to attack the carbocation (opposites attract!), but much more time for the I- to decide to leave the substrate. So, the identity and concentration of the substrate affects the reaction rate.

E1 Mechanism

E1 stands for 'elimination, 1st order.' The first step of an E1 reaction is identical to SN1: the substrate forms a carbocation. However, instead of an anion attacking this ion, an additional atom or group is eliminated from the substrate, forming a double bond on the substrate. Typically, the eliminated atom is just hydrogen, and a base is usually extracts the hydrogen. Let's again consider 3-iodo-1-propene, reacting with OH-:

3iodo1propenereaction

Just as with SN1, a carbocation is first formed. The hydroxide ion can attack the C atom, or it can extract a hydrogen atom, which ultimately forces the electrons in the C-H bond to go somewhere: into a new double bond. SN1 reactions often occur in parallel with E1 reactions. How much of the substitution and elimination products formed depends on the conditions and exact molecules used.

SN2 Mechanism

SN2 stands for 'nucleophilic substitution, 2nd order'. Both the nucleophile and electrophile affect the rate of reaction. In SN2 reactions, there is no carbocation formed. Instead, the nucleophile directly attacks the electrophilic atom on the substrate. Let's consider the molecule 2-iodopropane as an example of a substrate, and again we will add OH-:

sn2mechanismtosize

We can see that the nucleophile, I-, leaves the substrate in tandem with OH- attacking the central carbon atom on the substrate. As this occurs, the methyl groups and hydrogen on the attacked carbon must re-orient to make room for the incoming OH- ion.

Not all substrates can provide plenty of room for the substrate to attack the carbon attached to the leaving anion. For example, if the carbon bonded to the I- had three carbon substituents (for example, three CH3 groups), the OH- couldn't easily reach the carbon, and a slow reaction or no reaction would occur.

Additionally, since the nucleophile is attacking a neutral molecule, it's not nearly as quick as a cation and anion coming together, as in SN1 mechanisms. Larger anions tend to be better nucleophiles and lead to quick reaction, so I- is better than Br-, for example.

For SN2 reactions, the rate therefore depends on both substrate reorientation and concentration, as well as the identity and concentration of the incoming nucleophile.

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