Which is a stronger base, a secondary or tertiary amine

[csx]

im not sure why but i keep getting contradictory answers ughh

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[popopopop]

*edit* I concede to justadream, 2nd > 3rd in base strength.

 

[echoyjeff222]

tertiary -- you can think of it as hyperconjugation (electron donating) R groups --> more e- density

 

[JMMTB]

im not sure why but i keep getting contradictory answers ughh

Tertiary amines are more basic because the alkyl groups are electron donating. Bases are stronger if there is a greater electron density/concentration. With more alkyl groups present, the basicity goes up.

An important point to make is that this could make a Nucleophile worse though. This is why bulky bases are great for E2 reactions, but not for SN2 reactions.

 

[popopopop]

From wiki:
Like ammonia, amines are bases. Compared to alkali metal hydroxides, amines are weaker (see table for examples of conjugate acid Ka values). The basicity of amines depends on:

1. The electronic properties of the substituents (alkyl groups enhance the basicity, aryl groups diminish it).
2. Steric hindrance offered by the groups on nitrogen.
3. The degree of solvation of the protonated amine.

 

[csx]

The reason i got tripped up is b/c i could have sworn i read in a passage that 2>3=1> ammonia in terms of bascity.

 

[justadream]

@csx

The answer is secondary > tertiary.

In general, greater substitution helps to stabilize the positive charge (formed when you add a Hydrogen). However, you also have to consider sterics. Tertiary causes too much steric hindrance.

This is from Wade Organic Chemistry 8th edition.

 

[csx]

@csx

The answer is secondary > tertiary.

In general, greater substitution helps to stabilize the positive charge (formed when you add a Hydrogen). However, you also have to consider sterics. Tertiary causes too much steric hindrance.

This is from Wade Organic Chemistry 8th edition.

yooo bossmans got me again!

 

[popopopop]

@csx

The answer is secondary > tertiary.

In general, greater substitution helps to stabilize the positive charge (formed when you add a Hydrogen). However, you also have to consider sterics. Tertiary causes too much steric hindrance.

This is from Wade Organic Chemistry 8th edition.

Wiki says there are a few more factors involved. Your example is also in the chart in wiki, dimethylamine has a higher Kb than trimethylamine due to steric hindrance, which would make what you said correct, but then it goes on to say....

"+I effect of alkyl groups raises the energy of the lone pair of electrons, thus elevating the basicity. Thus the basicity of an amine may be expected to increase with the number of alkyl groups on the amine. However, there is no strict trend in this regard, as basicity is also governed by other factors mentioned above. Consider the Kb values of the methyl amines given above. The increase in Kb from methylamine to dimethylamine may be attributed to +I effect; however, there is a decrease from dimethylamine to trimethyl amine due to the predominance of steric hindrance offered by the three methyl groups to the approaching Brönsted acid."

 

[DrDreams]

As you increase alkyl groups on ammonia, it starts becoming a better nucleophile. A primary amine is a better base than ammonia, but a secondary amine is a better nucleophile than a primary amine, but a primary amine is a better base than a secondary one. A tertiary amine is just big and is sterically hindered as everyone has said. This is from my TBR study. Someone correct me if I'm wrong.

 

[justadream]

Just to clarify @DrDreams 's post (I think he made a typo with "but a secondary amine is a better nucleophile than a primary amine, but a primary amine is a better base than a secondary one.")

I think he meant to say:

Secondary > Primary > Tertiary

 

[DrDreams]

I will have to double check my notes, my brain is exhausted from taking a FL. Just finished. BUT, I remember it was saying that a primary was a better base, but the secondary was a better nucleophile than base.

In otherwords, a secondary amine will tend to behave more like a nucleophile (it will attack) and a primary amine will abstract a proton. Thats the sense I got from it. I shall go home and re-confirm it soon!

 

[justadream]

Wade Organic Chemistry 8th ed

 

[popopopop]

Wade Organic Chemistry 8th ed

Ok, I concede, SECONDARY > TERTIARY in alkalinity.

http://research.cm.utexas.edu/nbauld/CHAPTER 21.htm
http://www.wikipremed.com/mcat_course.php?code=0301050201000000
"In summary, while secondary ammonium cations aren't as inductively stabilized as tertiary ammonium cations, the difference is more than made for up by the solvation of the secondary ammonium cation by water molecules."

 

[hueso]

aqueous: secondary most basic
gas phase: tertiary most basic

 

[csx]

since we are on topic, what is the nucleophilic order or strength??