What is the major product of the reaction of a geminal dibromide with silver nitrate?

Question

$\ce{Ag+/H2O}$ is used for peroxide formation, then followed by hydrolysis to yield an alcohol. But in the question it is a geminal dihalide and I don't know if the mechanism follows through a carbocation intermediate.

According to the solution it should go through carbocation rearrangement, because the given answer is (b) which is obtained from ring expansion.

possible duplicate of https://chemistry.stackexchange.com/questions/41500/reaction-of-furan-with-chloroform-in-base .Check the answer in the link.That might help you — Chakravarthy Kalyan, Jun 08 '19 at 16:58
check ring exoansion of gem dihalo molecule in the same post. — Chakravarthy Kalyan, Jun 09 '19 at 02:36
@ChakravarthyKalyan - I'm not an organic chemist, but that carbene/carbanion mechanism looks very different to the carbocation mechanism here. — Karsten, Jun 09 '19 at 13:21
@ Karsten Theis ,yes mechanism is via carbocation assisted by Silver ion.One above is base catalysed via carbanion.I have answered this to give relevant mechanism. You are pretty good in understanding reaction mechanism.Pl.check my answer below. — Chakravarthy Kalyan, Jun 09 '19 at 13:42

score 1 · Accepted Answer · answered Jun 08 '19 at 21:13

This is a specific example of the Silver Nitrate Test, one of the experiments used in organic chemistry teaching laboratories. Usually, the Silver Nitrate Test allows for the identification of alkyl halides by observing them in an alcoholic silver nitrate environment. For change, here used aqueous environment instead. In general, You will test the reactivity of several alkyl halides in a $\mathrm{S_N1}$ reaction. The silver ion (a halophilic reactant) complexes with the halide and precipitates out of solution first (note that silver nitrate in aqueous or alcoholic solution promotes ionization of the alkyl halide), and resulting carbocation react with ethanol to form ethyl ethers (in your case, it forms alcohol with water). The rate of the silver halide salt precipitation is characteristic of different types of alkyl halides ($3^\circ \gt 2^\circ \gt 1^\circ$).

When silver nitrate is used with $1^\circ$- or $2^\circ$-alkyl halides, a rearrangement may occur before the product formation stage. For example:

$$\ce{(CH3)3CCH2-Br + H2O + AgNO3 -> (CH3)2C(OH)CH2CH3 + AgBr + HNO3}$$

Rearrangements will only occur when the resulting carbocation is more stable than the initial carbocation. For example, see a relevant reaction below (Ref.1):

In your case, $\ce{Ag+}$ ion act as a Lewis acid and abstract one of bromide to leave bromocyclopropyl carbocation which is also comparatively stable cation (see the ring expansion in the diagram). The driving force here to rearrange the structure is releasing the strain energy, which gives you a relatively stable allyl cylohexenyl carbocation. It would then react with a water molecule to give the compound 2 as the final product. The same result was obtain from this reaction and has been reported (Ref.2), abstract of which states that:

Several ring expansion products carrying vinylic bromo functionality were synthesized by opening of the geminal dibromobicyclo[n.1.0]alkanes ring. Dibromocarbene was formed from bromoform and potassium tert-butoxide in hexane. Its reaction with various cyclic alkenes was the resultant dibromobicyclo[n.1.0]alkanes. Then, opening was performed using $\ce{AgNO3}$ in various solvent systems, such as acetic acid/DMSO, acetic acid/DMF, $\ce{CH3OH}$/acetone, and $\ce{H2O}$/DMF.

References:

Russell J. Hewitta, Joanne E. Harvey, “Synthesis of C-furanosides from a D-glucal-derived cyclopropane through a ring-expansion/ring-contraction sequence,” Chem. Comm. 2011, 47(1), 421–423 (DOI: 10.1039/C0CC02244F).
Mesut Boz, Hafize Çalişkan, Ömer Zaím, “Silver Ion-Assisted Ring Expansions in Different Solvent Systems,” Turk. J. Chem. 2009, 33(1), 73–78 (DOI: 10.3906/kim-0807-3).

@ Mathew Mahidaratne ,quoting from your post "The driving force here to rearrange the structure is releasing the strain energy" , is logical explanation for ring expansion from 5-6.In example, in Ref.1,ring contraction from 7-5 seems opposite.Any mechanism for this change would help. — Chakravarthy Kalyan, Jun 09 '19 at 05:52
The ring expansion I mentioned is cyclopropyl ring. Ring opening gives more stable cyclohexene instead. In *Turk. Chem. examples, appropriate ring opening to give even larger ring, because releasing strain energy is important. — Mathew Mahindaratne, Jun 09 '19 at 19:59

score 1 · Answer 2 · edited Jun 10 '20 at 14:04

The question is summarised in the paper, Silver Ion-Assisted Ring Expansions in Different Solvent Systems.$\ce{^1}$

The same reaction is reported$\ce{^2}$,which states that:

Opening 6,6-dibromobicyclo[3.1.0]hexane (7) in a methanol/acetone solvent system resulted in 1-bromo6-methoxy-1-cyclohexene (9) 16 and 1,6-dibromo-1-cyclohexene (10).

Another solvent system (H2 O/DMF) was also used for ring-opening reactions of the dibromocyclopropane ring that was synthesized from branched alkenes and only gave the alcohol derivatives, 2-bromo-1-methylcyclohept-2-en-1-ol (13) and 2-bromo-3-methylcyclohept-2-en-1-ol (14).

The possible mechanism for this ring expansion is shown below.

Referances

1 . Mesut Boz, Hafize C¸Alis¸Kan, Omer Zaim ," Silver Ion-Assisted Ring Expansions in Different Solvent Systems,” Turk. J. Chem. 2009, 33(1), 73–78.

2 . Mesut Boz, Hafize C¸Alis¸Kan, Omer Zaim ," Silver Ion-Assisted Ring Expansions in Different Solvent Systems,” Turk. J. Chem. 2009, 33(1), 73–78.

What is the major product of the reaction of a geminal dibromide with silver nitrate?

2 Answers2