Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: pesona3f on April 21, 2009, 06:12:04 AM
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hello, i have a question... we know that primary amine basicity is lower compared to secondary but tertiary is lower han secondary due to steric effect, so.. how about primary and secondary, which is higher basicity??
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I think the sequence would be 2 > 3 > 1
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is there any reason for that? ???
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2>1>3>ammonia
in general
Secondary more than primary because of greater +I effect due to the 2 alkyl groups.
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can you explain bout the +1effect?? i dont get it clearly...
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Secondary amine suffers less steric hindrance than tertiary, and its lone pair is pushed out by two electron donating groups. Argument is similar for the primary amine but you must take note that steric hindrance only plays a major role once R-Groups get very big. Otherwise, we look at empirical data to say whether or not it is important or not.
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can you explain bout the +1effect?? i dont get it clearly...
What don't you understand about it?
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i mean, whats is I+ effect? is it the reason you say in general 1>3
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No. It is the reason I say in general 2>1.
See here.
http://en.wikipedia.org/wiki/Inductive_effect (http://en.wikipedia.org/wiki/Inductive_effect)
http://www.chemguide.co.uk/basicorg/bonding/eneg.html (http://www.chemguide.co.uk/basicorg/bonding/eneg.html) further down.
Inductive effect alone, in fact predicts 3>2>1
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i get it, but still i'm confuse why you say in general 1>3
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Draw the structure of a tertiary amine and a primary amine.
Suppose a proton approaches these molecules. It does not have much space to accept the lone pair on the nitrogen atom in the tertiary amine as compared to the primary amine. The molecule is "crowded", it is "sterically" hindered.
Approach is comparatively easier in case of the primary amine.
So, who is more basic?
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in that case of cos primary is easy to access because less steric effect. but, from the book i read it show that triethylamine pka is 10.76 while ethyl amine pka is 10.75. i wonder bout that?
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with respect to basicity of secondary and tertiary amines, I have a question:
does it hold true for the compounds bupropion (secondary amine) and diethylpropion as well?
http://en.wikipedia.org/wiki/Bupropion
http://en.wikipedia.org/wiki/Diethylcathinone
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Secondary amine suffers less steric hindrance than tertiary, and its lone pair is pushed out by two electron donating groups. Argument is similar for the primary amine but you must take note that steric hindrance only plays a major role once R-Groups get very big. Otherwise, we look at empirical data to say whether or not it is important or not.
I disagree with the steric hinderance argument, or at least that it is the only factor involved. The basicity of amines in non-protic solvents does follow the 3>2>1 order predicted on the grounds of inductive effects. The 2>1>3 order is observed in aqueous solution and is, as far as I was led to believe, a solvation effect. The ammonium ions of less substituted amines are better stabilised by hydrogen bonding (as there are more hydrogens bonded to N). So, solvation favours the protonation of the less substituted and induction favours the protonation of the more substituted. Solvation becomes the key factor for tertiary amines and causes the deviation from order of basicity predicted by induction.
I found a web page: http://www.cliffsnotes.com/WileyCDA/CliffsReviewTopic/Introduction-to-Amines.topicArticleId-23297,articleId-23288.html
This will also be explained in almost any organic chemistry textbook.