Any time a new bond is formed, take a look at the atom that’s accepting the lone pair of electrons. Does it have a full shell of electrons? Oftentimes it does.  If so, that means you’re going to have to break a bond that’s attached to that atom. If it’s a single bond you’re breaking, what you end up creating is called a leaving group.

Note how the second arrow always shows a pair of electrons going toward the leaving group. That means the charge on it is going to become more negative by 1 when it leaves. So if the leaving group is positively charged, it will become neutral, and if it’s neutral, it will become negative.

The identity of the leaving group is crucial to whether the reaction will happen at all.  There are “good” leaving groups and there are “bad” leaving groups. Reactions are more likely to take place when you can displace a good leaving group.

What makes a leaving group “good” or “bad”? Thankfully, there’s one simplifying factor to look at when deciding this: its basicity. Good leaving groups are weak bases.

 

How do we know what are weak bases? There’s a useful tool for that – it’s called a pKa table. Many pKa tables only specifically give you the identity of the acid, but if you think about it, it also gives you information about the conjugate base of each acid. The conjugate base is the part left over when you lose H+. The stronger the acid, the weaker the conjugate base.

And the weaker the conjugate base, the better the leaving group. So a pKa table is a great guide to leaving group ability.

One word of caution: pKa measures an equilibrium, whereas leaving group ability is based on reaction rates. So although the correlation is very good, it isn’t perfect.

Here’s a pKa table, annotated.


The trend is pretty clear – in general, the weaker the base, the better the leaving group. Furthermore, note how we (almost) never see alkanes or hydrogens as leaving groups. That’s because they’re strongly basic anions – and very unstable.

You might note that I have carefully avoided discussing fluorine. Fluorine tends to be a very poor leaving group for SN1/SN2/E1/E2 reactions. In Org 2, you may see some examples where F can act as a leaving group when it is attached to a carbonyl carbon or an aromatic ring. These reactions (addition-elimination reactions) are a little bit different in that the rate determining step is not so related to loss of the leaving group. There are some extra factors at work in these situations that we can discuss if you’re curious.

Next Post: Leaving Groups Are Nucleophiles Acting In Reverse

Related Posts:

{ 31 comments… read them below or add one }

kabtq9s August 11, 2011 at 6:36 pm

This is gold !!.. Thanks alot

Reply

James August 12, 2011 at 2:11 pm

Thanks!

Reply

Denise Lech September 28, 2011 at 10:11 pm

Awesome! This clarifies a lot! Thanks!!

Reply

james September 28, 2011 at 10:16 pm

Glad you found it useful!

Reply

zohra October 9, 2011 at 6:36 pm

I love your site!!! I have been on this more than my text, increasing my quiz score.

Thank you so much!

Reply

james October 10, 2011 at 3:01 pm

Glad you find it useful!

Reply

kb October 27, 2011 at 5:32 am

this was super clear!

Reply

samy December 20, 2011 at 6:00 am

you’re too awesome, thnx a million!

Reply

wesam January 7, 2012 at 9:39 pm

Which is the good living group phenoxy ( PhO) or chloride (Cl)?

Reply

james January 8, 2012 at 2:33 am

What’s a weaker base?

Reply

wesam January 10, 2012 at 1:49 pm

Which is the good leaving group phenoxy (PhO) or Chloride Cl?

Reply

james January 15, 2012 at 2:21 am

Procedure for you figuring this out for yourself: Go to a pKa table and figure out the pKa of Phenol (PhOH) and HCl. Which is a stronger acid? Now: the stronger the acid, the weaker the conjugate base. . So which of these would have the weaker conjugate base? The weaker the base, the better the leaving group.

Reply

Kranthi July 29, 2012 at 6:42 pm

phenol has a pKa of about 16 and HCl has a pKa of about -5 so “strong acid weak conjugate base” would imply that Cl- is the better leaving group.

Reply

Anon June 24, 2013 at 6:05 pm

anyone knows HCl is a strong acid!! STUPID

anon June 24, 2013 at 6:10 pm

hcl is a strong acid and it is the better leaving group

how did you get through general chemistry not knowing that?!

rachel January 14, 2014 at 10:40 pm

phenol at about 10 pka, HCl at about -7 pka. So HCl is the stronger acid=weaker conj. base = more stable as an anion = better leaving group. Do I have this thought process correct?

Reply

Ivana March 9, 2012 at 5:48 pm

The most understandable way I have ever seen! Thank you very much, please keep up the good work.

Reply

james March 12, 2012 at 12:17 am

Glad you find it helpful Ivana! thanks

Reply

Kit April 12, 2012 at 3:23 am

Thank you so much. Very lucid and informative. Please keep it up.

Reply

andre April 27, 2012 at 8:43 am

Awesome blog!!!

I have my MCAT exam in 2 days, (well 1.5 days if you start counting from now), wish I stumbled on your blog 3 months ago! well, still 1.5 x 12 hours left! :)

Reply

james April 27, 2012 at 5:32 pm

Glad you’re finding it useful. When you’re done, tell your friends :-)

Reply

Marina May 2, 2012 at 5:17 pm

Yes, please discuss the extra factors you mentioned regarding F- in a-e rxns.

Thanks so much for always making it clear and interesting!

Reply

Danielle November 26, 2012 at 1:49 pm

What happens if the leaving group is positive, making the charge on the carbon neutral, like S+(CH3)2?

Reply

james November 26, 2012 at 5:36 pm

Ah… well the leaving group would be S(CH3)2 which is a very weak base and an excellent leaving group ! (much like OH2(+)]

Reply

Earnest December 19, 2012 at 9:32 pm

This is just what I need,Organic chemistry has been a part of chemistry that seemed to be impossible to understand. But with the help of your explanations I see my self getting a straight A,beyond the shadow of doubt. Thanks to you James.

Reply

Joe Shmoe June 24, 2013 at 6:06 pm

this is a great website, veyr helpful and much clearer than my text or the professor. thank you.

Reply

kaitlyn October 26, 2013 at 4:11 pm

Thanks for this amazing resource to help me through orgo. It’s people like you who need to be in Universities teaching students because anyone can see based off this website you’ve made, that you actually care about the people learning and understanding orgo….thank you.

Reply

Nick November 14, 2013 at 9:55 am

Just wanted to say thank you. Wish I had found this website months ago. All the information is so accessible and very well organized. Keep up the tremendous work. You’ve gained a loyal visitor to your website. I’ll be back next semester for organic II as well.

Reply

Sonia November 18, 2013 at 3:17 pm

This website is so useful! Thank you very much!

Reply

K July 10, 2014 at 9:23 pm

There is a mistake in the second paragraph, I think. An atom can never be neutral and charged at the same time. A neutral atom’s atomic number(number of protons) is the same as the number of electrons. I recommend you to say an atom becomes an ion(negatively charged) to reach the full octect.

Reply

Lorraine October 22, 2014 at 4:15 pm

keep up your good work. your site really helped me and my friends most of the concepts in organic chemistry.

Reply

Leave a Comment