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

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{ 37 comments… read them below or add one }

kabtq9s

This is gold !!.. Thanks alot

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James

Thanks!

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Denise Lech

Awesome! This clarifies a lot! Thanks!!

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james

Glad you found it useful!

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zohra

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

Thank you so much!

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james

Glad you find it useful!

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kb

this was super clear!

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samy

you’re too awesome, thnx a million!

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wesam

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

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james

What’s a weaker base?

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wesam

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

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james

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.

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Kranthi

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.

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Anon

anyone knows HCl is a strong acid!! STUPID

anon

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

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

rachel

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?

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Ivana

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

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james

Glad you find it helpful Ivana! thanks

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Kit

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

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andre

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! :)

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james

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

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Marina

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

Thanks so much for always making it clear and interesting!

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Danielle

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

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james

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

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Earnest

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.

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Joe Shmoe

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

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kaitlyn

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.

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Nick

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.

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Sonia

This website is so useful! Thank you very much!

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K

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.

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Lorraine

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

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Adityam Ghosh

Is CN- a good leaving group

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James

No, it’s quite a poor leaving group

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lama

Thanks! But can u explain plz why weak bases are better leaving groups?

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Vedant

Where would O-Ph rank as a leaving group? Is it better than the halides?

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sahar

the site is excellent, thank you very much, you had solved most of my problems in organic chemistry.

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Varun

I ve seen alkoxides getting removed during sn2th reactions (related to carbonyls) . I’m in a mess and get choked up in such reactions.please help

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