Introduction to Nucleophilic Substitution Reactions

by James

in Chemical Bonds, Organic Chemistry 1, Organic Reactions

Following up on the 4 most important patters of reactions in Org 1, and introduction to acid-base reactions, here’s the second major pattern. It’s called nucleophilic substitution. I won’t go into nucleophiles and electrophiles in this post. The point is actually not to understand what’s going on here!

Don’t understand the reactions below. Just focus on being able to see one thing: what bonds are formed and what bonds are broken. 

Just that one thing. Later on we’ll go into more detail about why things happen. But not today.

Here are three examples of nucleophilic substitution reactions.

What’s the pattern? In each case, we’re breaking a bond at carbon, and forming a new bond at carbon. (and yes, salts form too… but this is organic chemistry, so we’re carbo-centric.)

This is an extremely common pattern for reactions and you will see it over and over again in Org 1 and Org 2.

Now let’s set the stage for later discussion. Here’s some interesting results that experiments tell us. We don’t get this information by thinking about what happens and predicting – we have to interrogate nature in order to get her to give up her secrets.

Interesting observation #1: when we measure the reaction rate.  In some substitution reactions the rate is proportional to  the concentration of two different species (e.g. the alcohol and HBr). In other cases, it only depends on the concentration of one species (i.e. the alcohol). Interesting!


 Interesting observation #2: In certain cases (like the reaction below) the reaction rate depends on the type of alkyl halide. Primary alkyl halides are slow, but tertiary alkyl halides are fast.

Interesting observation #3: In other cases, the reaction rate depends on the type of alkyl halide, but it is the primary alkyl halide that is fast. And the tertiary alkyl halide does not even give substitution products… you’ll notice the double bond there. This  is actually an elimination reaction.

Interesting observation #4: Finally, there is a property that *some* molecules have of rotating plane polarized light. It’s been known since the late 19th century. In some substitution reactions, molecules that are “optically active” retain their optical activity in the reaction… but in others, this “optical activity” disappears.

What’s going on? How can we come up with a hypothesis for why and how these reactions work?

Next Post In The Series: Introduction To Addition Reactions

Related Posts:

{ 5 comments… read them below or add one }

John Spevacek September 26, 2011 at 12:19 pm

(Psst. – The second equation under example #1 is incorrect, unless you intended to show that no reaction occurs.)

Reply

james September 26, 2011 at 12:32 pm

Thanks! Fixed

Reply

goshu abraham April 12, 2012 at 1:28 pm

very nice!!

Reply

Pleza January 26, 2013 at 10:06 am

Please give little more explanation regarding example #3

Reply

james January 27, 2013 at 10:49 pm

This is a classic example of an E2 being favored over SN2, see this post (and those previous) – http://masterorganicchemistry.com/2013/01/18/wrapup-the-quick-n-dirty-guide-to-sn1sn2e1e2/

Reply

Leave a Comment