Reactions of Alkyl Halides with Reducing Metals
The alkali metals (Li, Na, K etc.) and the alkaline earth metals (Mg and Ca, together with Zn) are good reducing agents, the former being stronger than the latter. Sodium, for example, reduces elemental chlorine to chloride anion (sodium is oxidized to its cation), as do the other metals under varying conditions. In a similar fashion these same metals reduce the carbon-halogen bonds of alkyl halides. The halogen is converted to halide anion, and the carbon bonds to the metal (the carbon has carbanionic character). Halide reactivity increases in the order: Cl < Br < I. The following equations illustrate these reactions for the commonly used metals lithium and magnesium (R may be hydrogen or alkyl groups in any combination). The alkyl magnesium halides described in the second reaction are called Grignard Reagents after the French chemist, Victor Grignard, who discovered them. The other metals mentioned above react in a similar manner, but the two shown here are the most widely used. Although the formulas drawn here for the alkyl lithium and Grignard reagents reflect the stoichiometry of the reactions and are widely used in the chemical literature, they do not accurately depict the structural nature of these remarkable substances. Mixtures of polymeric and other associated and complexed species are in equilibrium under the conditions normally used for their preparation.
R3C-X + 2Li ——> R3C-Li + LiX An Alkyl Lithium Reagent
X + Mg ——> R3C-MgX A Grignard Reagent
The metals referred to here are insoluble in most organic solvents, hence these reactions are clearly heterogeneous, i.e. take place on the metal surface. The conditions necessary to achieve a successful reaction are critical.
First, the metal must be clean and finely divided so as to provide the largest possible surface area for reaction.
Second, a suitable solvent must be used. For alkyl lithium formation pentane, hexane or ethyl ether may be used; but ethyl ether or THF are essential for Grignard reagent formation.
Third, since these organometallic compounds are very reactive, contaminants such as water, alcohols and oxygen must be avoided.
These reactions are obviously substitution reactions, but they cannot be classified as nucleophilic substitutions, as were the earlier reactions of alkyl halides. Because the functional carbon atom has been reduced, the polarity of the resulting functional group is inverted (an originally electrophilic carbon becomes nucleophilic). This change, shown below, makes alkyl lithium and Grignard reagents unique and useful reactants in synthesis.
Reactions of organolithium and Grignard reagents reflect the nucleophilic (and basic) character of the functional carbon in these compounds. Many examples of such reactions will be encountered in future discussions, and five simple examples are shown below. The first and third equations demonstrate the strongly basic nature of these compounds, which bond rapidly to the weakly acidic protons of water and methyl alcohol (colored blue). The nucleophilic carbon of these reagents also bonds readily with electrophiles such as iodine (second equation) and carbon dioxide (fifth equation). The polarity of the carbon-oxygen double bonds of CO2 makes the carbon atom electrophilic, shown by the formula in the shaded box, so the nucleophilic carbon of the Grignard reagent bonds to this site. As noted above, solutions of these reagents must also be protected from oxygen, since peroxides are formed (equation 4).
Another important reaction exhibited by these organometallic reagents is metal exchange. In the first example below, methyl lithium reacts with cuprous iodide to give a lithium dimethylcopper reagent, which is referred to as a Gilman reagent. Other alkyl lithiums give similar Gilman reagents. A useful application of these reagents is their ability to couple with alkyl, vinyl and aryl iodides, as shown in the second equation. Later we shall find that Gilman reagents also display useful carbon-carbon bond forming reactions with conjugated enones and with acyl chlorides.
2 CH3Li + CuI ——> (CH3)2CuLi + LiI Formation of a Gilman Reagent
(C3H7)2CuLi + C6H5I
——> C6H5-C3H7 + LiI + C3H7Cu A Coupling Reaction
The formation of organometallic reagents from alkyl halides is more tolerant of structural variation than were the nucleophilic substitutions described earlier. Changes in carbon hybridization have little effect on the reaction, and 1º, 2º and 3º-alkyl halides all react in the same manner. One restriction, of course, is the necessary absence of incompatible functional groups elsewhere in the reactant molecule. For example, 5-bromo-1-pentanol fails to give a Grignard reagent (or a lithium reagent) because the hydroxyl group protonates this reactive function as soon as it is formed.
BrCH2CH2CH2CH2CH2OH + Mg ——> [ BrMgCH2CH2CH2CH2CH2OH ] ——> HCH2CH2CH2CH2CH2OMgBr
Exchange metalation is particularly useful when it can be directed to specific sites in a molecule. One such case is the directed ortho metalation of aromatic rings bearing a suitable directing group.
Reactions of Dihalides
Reactions of Dihalides
If two halogen atoms are present in a given compound, reactions with reducing metals may take different paths depending on how close the carbon-halogen bonds are to each other. If they are separated by four or more carbons, as in the first example below, a bis-organometallic compound may be formed. However, if the halogens are bonded to adjacent (vicinal) carbons, an elimination takes place with formation of a double bond. Since vicinal-dihalides are usually made by adding a halogen to a double bond, this reaction is mainly useful for relating structures to each other. The last example, in which two halogens are bonded to the same carbon, referred to as geminal (twinned), gives an unusual reagent which may either react as a carbon nucleophile or, by elimination, as a carbene. Such reagents are often termed carbenoid.
The solution structure of the Simmons-Smith reagent is less well understood than that of the Grignard reagent, but the formula given here is as useful as any that have been proposed. Other alpha-halogenated organometallic reagents, such as ClCH2Li, BrCH2Li, Cl2CHLi and Cl3CLi, have been prepared, but they are substantially less stable and must be maintained at very low temperature (ca. -100 º C) to avoid loss of LiX. The stability and usefulness of the Simmons-Smith reagent may be attributed in part to the higher covalency of the carbon-zinc bond together with solvation and internal coordination of the zinc. Hydrolysis (reaction with water) gives methyl iodide, confirming the basicity of the carbon; and reaction with alkenes gives cyclopropane derivatives, demonstrating the carbene-like nature of the reagent. The latter transformation is illustrated by the equation on the right.
Elimination reactions of the stereoisomeric 1,2-dibromo-1,2-diphenylethanes provide a nice summary of the principles discussed above. The following illustration shows first the meso-diastereomer and below it one enantiomer of the racemic-diastereomer. In each case two conformers are drawn within parentheses, and the anti-relationship of selected vicinal groups in each is colored green. The reaction proceeding to the left is a dehydrohalogenation induced by treatment with KOH in alcohol. Since this is a stereospecific elimination, each diastereomer gives a different stereoisomeric product. The reaction to the right is a dehalogenation (the reverse of halogen addition to an alkene), caused by treatment with iodide anion. Zinc dust effects the same reaction, but with a lower degree of stereospecificity. The mechanism of the iodide anion reaction is shown by red arrows in the top example. A similar mechanism explains the comparable elimination of the racemic isomer. In both reactions an anti-transition state is observed.
The two stereoisomers of 1-bromo-1,2-diphenylethene (shown on the left of the diagram) undergo a second dehydrobromination reaction on more vigorous treatment with base, as shown in the following equation. This elimination generates the same alkyne (carbon-carbon triple bond) from each of the bromo-alkenes. Interestingly, the (Z)-isomer (lower structure) eliminates more rapidly than the (E)-isomer (upper structure), again showing a preference for anti-orientation of eliminating groups.
C6H5CH=CBrC6H5 + KOH ——> C6H5C≡CC6H5 + KBr + H2O
Preparation of Alkynes by Dehydrohalogenation
The last reaction shown above suggests that alkynes might be prepared from alkenes by a two stage procedure, consisting first of chlorine or bromine addition to the double bond, and secondly a base induced double dehydrohalogenation. For example, reaction of 1-butene with bromine would give 1,2-dibromobutane, and on treatment with base this vicinal dibromide would be expected to yield 1-bromo-1-butene followed by a second elimination to 1-butyne.
CH3CH2CH=CH2 + Br2 ——> CH3CH2CHBr–CH2Br + base ——> CH3CH2CH=CHBr + base ——> CH3CH2C≡CH
In practice this strategy works, but it requires care in the selection of the base and solvent. If KOH in alcohol is used, the first elimination is much faster than the second, so the bromoalkene may be isolated if desired. Under more extreme conditions the second elimination takes place, but isomerization of the triple bond also occurs, with the more stable isomer (2-butyne) being formed along with 1-butyne, even becoming the chief product. To facilitate the second elimination and avoid isomerization the very strong base sodium amide, NaNH2, may be used. Since ammonia is a much weaker acid than water (by a factor of 1018), its conjugate base is proportionally stronger than hydroxide anion (the conjugate base of water), and the elimination of HBr from the bromoalkene may be conducted at relatively low temperature. Also, the acidity of the sp-hybridized C-H bond of the terminal alkyne traps the initially formed 1-butyne in the form of its sodium salt.
CH3CH2C≡CH + NaNH2 ——> CH3CH2C≡C:(–) Na(+) + NH3
An additional complication of this procedure is that the 1-bromo-1-butene product of the first elimination (see previous equations) is accompanied by its 2-bromo-1-butene isomer, CH3CH2CBr=CH2, and elimination of HBr from this bromoalkene not only gives 1-butyne (base attack at C-1) but also 1,2-butadiene, CH3CH=C=CH2, by base attack at C-3. Dienes of this kind, in which the central carbon is sp-hybridized, are called allenes and are said to have cumulated double bonds. They are usually less stable than their alkyne isomers.
Hence, correct answer is alkyl iodide.What is the reactivity order of alkyl halide? ›
Therefore, the decreasing order of reactivity of alkyl halide is RI > RBr > RCl .How reactive are alkyl halides? ›
The alkyl halides are very reactive due to highly polarized \[CX\] bonds with a large difference in electronegativities of carbon and halogen atoms. As per the leaving ability, the order is \[I>Br>Cl>F\].What chemical reactivity does alkyl halides show? ›
Alkyl halides can undergo two major types of reactions - substitution and/or elimination. The substitution reaction is called a Nucleophilic Substitution reaction because the electrophilic alkyl halide forms a new bond with the nucleophile which substitutes for (replaces) the halogen at the alpha-carbon.What is the order of reactivity of halogens? ›
Order of reactivity towards halogenation is F2>Cl2>Br2>I2.Is primary alkyl halide most reactive? ›
From the above, it is clear that X atom is released as X– most readily in tertiary halides and least readily in primary halides. Hence, tertiary halides are most reactive, while primary halides are least reactive.Which alkyl halide is more reactive primary secondary or tertiary? ›
Now, we know that tertiary carbocation is much more stable than secondary and the secondary carbocation is much more stable than the primary due to the inductive effect of the alkyl groups. Hence for SN1 reaction, the order of reactivity of the alkyl halides is tertiary > secondary > primary.Which alkyl halide has highest reactivity for a particular alkyl group? ›
Which alkyl halide has the highest reactivity for a particular alkyl group? Explanation: Reactivity order for the alkyl halides towards Sn2 reaction is R-I>R-Br>R-Cl>R-F. This can be explained by which halogen atom is a better leaving group compared to the other. 3.What is the order of reactivity of alkyl halides towards SN1 and SN2 reaction? ›
3o carbocations are more stable than others or stability order of carbocations is 3o>2o>1o, so, order of alkyl halides towards SN1 reaction is 3o>2o>1o.What is the order of the reactivity series? ›
The order is:
The order of reactivity towards SN2 mechanism is methyl halide > primary alkyl halide > secondary alkyl halide > tertiary alkyl halide.Why alkyl halides are more reactive than alkanes? ›
Halogenoalkanes are based on alkanes so they have all single bonds and are therefore SP3 hybridized. With the exception of the fluroalkane they are more reactive than alkanes principally due to two reasons: 1. C-X bond is polar due to the difference in electronegativity between carbon and X.What is the order of reactivity of different alkyl halides in nucleophilic substitution? ›
The order of reactivity is iodide > bromides > chlorides > fluorides.Why alkyl halides are more reactive than aryl halides? ›
Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides is because of resonance stabilization in aryl halide. Due to resonance, C-X bond becomes shorter and stronger and cannot be easily replaced by nucleophiles.Which alkyl halide has highest boiling point? ›
Thus $1 - $ chlorobutane has the highest boiling point.Why are alkyl halides more reactive? ›
The high reactivity of alkyl halides can be explained in terms of the nature of C — X bond which is highly polarized covalent bond due to large difference in the electronegativities of carbon and halogen atoms.What is the order of reactivity of alkyl halides for SN1 reaction? ›
1. In the SN1 mechanism, tertiary alkyl halides are more reactive. A tertiary carbocation is more stable than a secondary carbocation which is more stable than a primary carbocation.Does reactivity increase down a group? ›
Reactivity of elements increases down the group as down the group number of shells increases and thus nuclear pull on the outermost electrons decreases.Which alkyl halide is more reactive towards nucleophile? ›
The order of reactivity is iodide > bromides > chlorides > fluorides.Why are tertiary alkyl halides less reactive in SN2? ›
The repulsion between the alkyl or any other groups present on an carbon atom, if the distance between the two is less than vander waals radius, than it is said to be the steric hindrance. That is the reason why tertiary alkyl halides are practically inert to substitution by SN2 mechanism as there is steric hindrance.
So, CH2 = CH.CH2.Cl is most reactive halide.Which alkyl halide is most stable? ›
So, CF4 is the most stable halide among the given options.Why are primary alkyl halides more reactive in SN2? ›
Primary alkyl halides undergo SN2 mechanisms because 1∘ substrates have little steric hindrance to nucleophilic attack and 1∘ carbocations are relatively unstable.Why tertiary alcohol are more reactive than primary? ›
The tertiary alcohol is more reactive than other alcohols because of the presence of the increased number of alkyl groups. These alkyl group increases the +I effect in the alcohol.Which alkyl halide has the least reactivity for a particular alkyl group? ›
This is Expert Verified Answer
To react with the alkyl halides, the carbon-halogen bond has got to be broken. Because that gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order. Iodoalkanes are the most reactive and fluoroalkanes are the least.
Thus 2-bromopropane is the most reactive.Which halide gives best SN2? ›
Methyl halides and 1° halides are the best at undergoing SN2 reactions, 2° halides are OK but 3° halides cannot go through the inversion process and will never do this reaction. The transition state is too crowded.Why are 3 alkyl halides more reactive in sn1 reactions? ›
Tertiary alkyl halides show SN1 reaction mostly. Why? Tertiary alkyl halides can ionize in an appropriate solvent producing tertiary carbocations—the first step in the SN1 mechanism. Tertiary carbocations are MORE stable than secondary or primary carbocations and are therefore easier to form.Which is the most reactive in sn1 reaction? ›
Solution : Most reactive towards `SN^(1)` reaction , the 3 degree halide is most reactive because it is stabilised by two phenyl group due to resonance. Step by step solution by experts to help you in doubt clearance & scoring excellent marks in exams.What is the reactivity order of SN2 reaction? ›
In general, the order of reactivity of alkyl halides in SN2 reactions is: methyl > 1° > 2°. The 3° alkyl halides are so crowded that they do not generally react by an SN2 mechanism.
The least reactive metals would be Platinum, Gold, Palladium, Osmium and Silver and in the decreasing order.Which of the following is most reactive? ›
Fluorine is the most reactive element among the given elements.What is the second most reactive metal? ›
The alkaline earth metals are the second most reactive family of elements. Beryllium, magnesium, calcium, strontium, barium and radium are all shiny, and silvery-white. They all have low densities, melting points and boiling points, and they tend to form solutions with a pH greater than 7.Which halogen reacts fastest in SN2? ›
Hence, CH3CH2I is most reactive towards SN2 reaction.Why alkyl halides are very reactive towards nucleophile? ›
Solution : The alkyl halides are very reactive due to highly polarised C-X bond with a large difference in electronegativities of carbon and halogen atoms.Why alkene is more reactive than alkane? ›
Alkenes are relatively stable compounds, but are more reactive than alkanes because of the reactivity of the carbon–carbon π-bond. Most reactions of alkenes involve additions to this π bond, forming new single bonds. The carbon-carbon double bond in alkenes such as ethene react with concentrated sulfuric acid.Why alkyl halide is more reactive than vinyl chloride? ›
Vinyl halides are less reactive than alkyl halides . This is because C-X bond in vinyl halides have partial double bond character due to resonance . So, it is difficult to break the C-X bond .How do you know which nucleophile is most reactive? ›
Solution : `CH_(3)-overset(Î˜)O` is sterically least hindered and hence most reactive nucleophile.Which alkyl halide has lowest boiling point? ›
Therefore tert-butyl chloride has the lowest boiling point.What is the order of reactivity of alkyl halides towards elimination reaction? ›
The order of reactivity of alkyl halides towards elimination reaction is. Solution : Order of reactivity of alkyl halides towards elimination reaction (both E1 and E2) is `3^(@) gt 2^(@) gt 1^(@)`.
Aryl halide is less reactive than alkyl halide towards nucleophilic substitution because of the resonance stabilization and sp2 hybridization of C attached to halide.Why allyl halides are more reactive than vinyl halides? ›
- This means that the electronegativity of carbon atoms in vinyl chloride is stronger than the carbon present in the allyl chloride due to which the bond will be stronger in the vinyl chloride. - Hence, due to weak bonds in allyl chloride, it will be more reactive.Why are aryl halides unreactive in SN1 and SN2? ›
Aryl halides are relatively unreactive toward nucleophilic substitution reactions. This lack of reactivity is due to several factors. Steric hindrance caused by the benzene ring of the aryl halide prevents S N2 reactions. Likewise, phenyl cations are unstable, thus making S N1 reactions impossible.Which alkyl halide is least polar? ›
Aryl halides tend to be less polar than alkyl halides (since an sp2 C is more electronegative than an sp3 C)Which is heavier alkyl halide or water? ›
Alkyl chlorides are generally lighter than water, while alkyl bromides and alkyl iodides are heavier than water. The order of density is RI > RBr > RCl. Poly chlorides are heavier than water. Thus the density of alkyl halides increases with the increase in the number and atomic mass of the halogen atoms.What is the order of boiling point of alkyl halides? ›
The boiling points of alkyl halides decrease in the order RI> RBr> RCl> RF.Which alkyl halide is more reactive primary secondary or tertiary? ›
In S(N^1) reactions, primary alkyl halides are more reactive than tertiary alkyl halides.Why are alkyl halides more reactive? ›
The high reactivity of alkyl halides can be explained in terms of the nature of C — X bond which is highly polarized covalent bond due to large difference in the electronegativities of carbon and halogen atoms.Why allyl halides are more reactive than alkyl halide? ›
allyl compoumd is more reactivbe because it form stable carbocation which is resonance stabilized so it is more reactive than n alkyl compounds towards nucleophilic reaction.Which one is more reactive alkyl halide or aryl halide? ›
Aryl halide is less reactive than alkyl halide towards nucleophilic substitution because of the resonance stabilization and sp2 hybridization of C attached to halide.
So, CF4 is the most stable halide among the given options.Which alkyl halide is more reactive in SN2 reaction? ›
The order of reactivity towards SN2 mechanism is methyl halide > primary alkyl halide > secondary alkyl halide > tertiary alkyl halide.Why are primary alkyl halides more reactive in SN2? ›
Primary alkyl halides undergo SN2 mechanisms because 1∘ substrates have little steric hindrance to nucleophilic attack and 1∘ carbocations are relatively unstable.Which alkyl halide has highest reactivity for a particular alkyl group? ›
Which alkyl halide has the highest reactivity for a particular alkyl group? Explanation: Reactivity order for the alkyl halides towards Sn2 reaction is R-I>R-Br>R-Cl>R-F. This can be explained by which halogen atom is a better leaving group compared to the other. 3.Which haloalkane is most reactive? ›
Thus 2-bromopropane is the most reactive.Why are alkyl halides more reactive than alkanes? ›
Halogenoalkanes are based on alkanes so they have all single bonds and are therefore SP3 hybridized. With the exception of the fluroalkane they are more reactive than alkanes principally due to two reasons: 1. C-X bond is polar due to the difference in electronegativity between carbon and X.Which is more reactive benzyl halide or aryl halide? ›
Benzyl halides are more reactive than vinyl and aryl halides.Why is benzyl halide more reactive? ›
Whereas in benzyl chloride, the loss of chlorine yields a benzyl carbocation, which is resonance stabilized. Hence it is more reactive.Why benzyl halide is more reactive than alkyl halides? ›
Benzyl chloride are far more reactive than alkyl halide towards nucleophilic substitution reaction due to the reason that the carbocation formed after the removal of halide ion is stabilized by resonance.Why are aryl halides less reactive? ›
Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides is because of resonance stabilization in aryl halide. Due to resonance, C-X bond becomes shorter and stronger and cannot be easily replaced by nucleophiles.
Aryl halides are relatively unreactive toward nucleophilic substitution reactions. This lack of reactivity is due to several factors. Steric hindrance caused by the benzene ring of the aryl halide prevents S N2 reactions. Likewise, phenyl cations are unstable, thus making S N1 reactions impossible.Which is more reactive allylic or benzylic? ›
Since the order of reactivity of alcohols towards HX is as follows: Benzyl > Allyl > 3° > 2° > 1°