Specific question from TPR Test 7

[kobe200LATE]

I have a question about this free standing question from a TPR FL test.

RX + 2Li --> RLi + LiX

In the formation of an organolithium agent from an alkyl halide and lithium metal:

A) One lithium is oxidized, the other is reduced
B) Both lithiums are oxidized
C) Both lithiums are reduced
D) Neither lithium is reduced or oxidized


So I went with A, but the answer is B. Can someone explain this for me because their explanation was really weak. Also, if both are getting oxidized, then what gets reduced?

Thanks,
kobe-2THOUSAND-AND-LATE

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[Cjc555777]

You're right, the book is wrong. Lots of times the test prep companies get these things wrong. If you do the oxidation numbers on the products side, the R group gets a +1, the halogen gets -1, and the Li gets 0. On the products side, the R and the X stay the same and the lithiums oxidize and reduce each other making one +1 and one -1. The companies are wrong a lot unfortunately. What's their explanation?

 

[kobe200LATE]

their explanation was literally "Both lithiums lose electrons"

so yea it was obviously of no help

 

[Cjc555777]

wow yeah, well their wrong. Just ignore what they said

 

[planktonelement]

Really? I think the book is correct. Both lithiums start off with 0 as their oxidation number. Then both of them have +1 in the products. The Li in LIX is pretty obvious. You can look at the R group as having gained an electron to form a organometallic nucleophile (like a Grignard reagent). The Li has to have a +1 charge on it to balance out the -1 of the R.

Someone correct me if I'm wrong...

 

[dundunham]

Hey dude, I find that TPR likes to do this kind of stuff. If you have the science workbook, look at number 3 of passage 17 in chemistry and you will see a similar type of question where the actual equation is unbalanced so it's not both reduction and oxidation happening... it's just reduction. I think this is what they were trying to do on that question.

 

[Chowdder]

I'm still a little tipsy from the beer I had tonight but I'll give you my input on this question (to the best of my ability at least). Don't worry, I got this question wrong too, but the book is right. Both lithium are being oxidized.

The reaction forms an organolithium reagent, which like the grinard reagent, is a oraganometallic compound. What you are doing is to create a carbanion, which is a powerful nucleophile and base that will alkylate other electrophiles.

This is not how the reaction proceeds, but I thought it helps to think of it this way. Turning R into a carbanion means that it will be the one that gains the pair of electrons. Therefore, if you give the electron pair to R from the original RX, then you will have X++, which as you all know is highly eletronegative. This will cause X to grab hold of electron from less electronegative atoms (i.e. Li). So the two Li are being oxidized to form LiX. In reality, what is being reduced, is the carbanion (which goes from being +1 to -1). Again the reaction does not proceed in that manner, but I just thought that it's easier to make sense of the reaction by thinking this way.

 

[kobe200LATE]

Okay its starting to make sense now. Also in RLi, Li is less electronegative than R so it should carry the higher oxidation state (+1) than the R group (-1). It wouldn't make sense for the Li to carry a -1 charge because that is too unstable.

 

[chemtopper]

RX + 2Li --> RLi + LiX
If you write it in ionic form it goes like this
R + X- + Li ----> R - Li + + Li+ X-
Charges are indicated by red color.
So you can see Li has positive charge in both the compounds .We can say it is oxidized only.
Do you remember Grignard reagent - RMgX
What is the charge on R here or Carbon bonded to Mg ? Isn,t negative?R-Mg2+X-
All organometallic compounds have C-Metal bond in which M has positive and c has negative charge on it.
We know that metals are more electropositive than carbon.
SO BOOK IS RIGHT.

 

[Aggro]

Yep, the book is correct. This is not a redox reaction.

As a general rule, alkali metals ALWAYS have an oxidation number of +1 when bound. The only exception are alkalides, but these are 99.99% guaranteed to not be on the MCAT.