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I came across this question where I had to convert $\ce{CH3-CH2-Mg-Br}$ to $\ce{CH3-CH2-NH2}$. If I need to do so can I just react the given compound with water to get ethane and then react it with $\ce{NH3}$.

If the answer is no then why can't I?
Is there any set of principles in organic chemistry which govern as to which substances can be replaced with what?
Kind of like the reactivity series of metals?
For e.g.: If I react an alkane with a halogen to get a haloalkane and then react this back with hydrogen to get the alkane then I don't think so it would be correct to do so. And I don't think that organic chemistry works that way.
But how do I understand while solving a question whether I am going in the right direction. Please help.

NB: This is not a homework question its just a question I stumbled across while my preparation for a college entrance test. (edit the title maybe)

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    $\begingroup$ A known method of producing primary amines directly from Grignard reagents is reaction with NH2Cl in ether. It makes more sense to convert the original alkyl bromide into the amine using a nitride/reducing agent or even the Gabriel synthesis. $\endgroup$
    – J. LS
    Commented Apr 1, 2015 at 22:13

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I came across this question where I had to convert $\ce{CH3-CH2-Mg-Br}$ to $\ce{CH3-CH2-NH2}$. If I need to do so can I just react the given compound with water [...]

Always be cautious about unwanted, side reactions that may occur. Proton exchanges are one huge class of possible side reactions. Protons transfers occur readily in many circumstances because the protons is small and can be shuffled around without much to get in its way, physically.

So in the case of your hypothetical reaction between a Grignard reagent and water - realize that Grignards are strongly basic and therefore will deprotonate water. Unfortunately protonated Grignards are no good in terms of reactivity. They just fall back apart into hydrocarbons and a basic halide salt.

Further reading regarding Grignards - It is tempting to use Grignards to create carbon-carbon bonds, but anything that is acidic as water or more acidic will be deprotonated by the Grignard before any carbon-carbon bonds are created. When preparing Grignards, glassware must be perfectly dry. Not just wiped dry with a paper towel but heated dry. Grignards also live in ether solvents since ethers are only very weakly acidic.

Grignards may be used to attack aldehydes and ketones in creating carbon-carbon bonds, but realize that if the aldehyde or ketone contains a functional group that is about as acidic as water or more so - then the first equivalent of Grignard you add isn't going to the aldehyde or ketone carbon - it's going to perform a proton-transfer.

Is there any set of principles in organic chemistry which govern as to which substances can be replaced with what?

In general, it is good to keep in mind the motif nucleophile attacks electrophile in introductory organic chemistry.

Exceptions to motif: nucleophile attacks electrophile

Other heuristics you can use in predicting what reactions might commence ...

  • Bond energies. This is mainly useful for studying radical reactions - to which the motif nucleophile attacks electrophile doesn't apply as well. Are we going from weaker to stronger bonds? That's a favorable move, enthalpically.
  • Polarities. This falls under nucleophile attacks electrophile.
  • Kinetics. It's good to learn the difference between thermodynamics and kinetics early in organic chemistry. Sometimes chemical entities may be thermodynamically unstable but kinetically stable. Maybe a carbon is particularly electrophilic, but it's too well "hidden" by neighboring groups (steric hindrance). So will a nucleophile attack this carbon? Probably not to a great extent.
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The short answer is no, you can't. Water will hydrolyze a Grignard reagent, destroying its reactivity. The product of the hydrolysis, according to this web site is an alkane and a basic magnesium bromide: $$\ce{CH3CH2MgBr + H2O -> CH3CH3 + MgBrOH}$$

So before you ever got to reacting anything with ammonia, your reactants would be lost as ethane gas bubbles out of your reaction.

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  • $\begingroup$ I agree to this.my other questions? $\endgroup$
    – Karan Singh
    Commented Mar 3, 2015 at 18:06
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Alkanes react only with very powerful oxidazing agents, like chlorine or oxygen, because they are very unreactive (due to their weak polarity).

Yes, there are such principles in organic chemistry and they are based most of the times on the reactivity of the substante. (acidity, polarity, aromaticity).

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Yes, a Grignard reagent can react with ammonia like the following:

$$\ce{RCH3Mg + NH3 -> R-H + MgNH3OH}$$

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  • $\begingroup$ I made a minimal modification of your equation but I couldn't actually get what you were trying to say. Could you edit it? And welcome to chemistry.SE! $\endgroup$
    – schneiderfelipe
    Commented Mar 27, 2017 at 12:40

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