nucleophiles

[donaldduck]

In an experiment students tried to synthesize an alkyl chloride by substituting HCl for HBr, and reacting it with ethanol but noticed the the rxn rate was sluggish. What is the reason for this decreased reactivity?

A. Chloride is a weaker nucleophile than bromide is
B. Chloride is a weaker electrophile than bromide is
C. Chloride stabilizes the intermediate carbocation that is produced
D. Chloride destabilizes the intermediate carbocation that is produced.

The answer is A

I picked B, because based from periodic trends chloride is a better nucleophile than bromine is and there wasnt any solvent specified in the problem or in the passage. I kno that since this is an sn2 rxn that the rate is dependent on the electrophile (ethanol) and the nucleophile (Cl-). but I dont understand how Cl- can possibly be a worse nucleophile than (Br-).

help!

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[AcclratngCharge]

In an experiment students tried to synthesize an alkyl chloride by substituting HCl for HBr, and reacting it with ethanol but noticed the the rxn rate was sluggish. What is the reason for this decreased reactivity?

A. Chloride is a weaker nucleophile than bromide is
B. Chloride is a weaker electrophile than bromide is
C. Chloride stabilizes the intermediate carbocation that is produced
D. Chloride destabilizes the intermediate carbocation that is produced.

The answer is A

I picked B, because based from periodic trends chloride is a better nucleophile than bromine is and there wasnt any solvent specified in the problem or in the passage. I kno that since this is an sn2 rxn that the rate is dependent on the electrophile (ethanol) and the nucleophile (Cl-). but I dont understand how Cl- can possibly be a better nucleophile than (Br-).

help!

Remember the periodic trends: elements down and to the left have more Nucleophilic behavior: Br is below Cl. Electronegativity reduces nucleophilicity. Nu bind to Carbon, not Hydrogen.

 

[donaldduck]

Remember the periodic trends: elements down and to the left have more Nucleophilic behavior: Br is below Cl. Electronegativity reduces nucleophilicity. Nu bind to Carbon, not Hydrogen.

I thought that since chlorine was smaller it has a greater charge density, making it a better nucleophile.

 

[PiBond]

In an experiment students tried to synthesize an alkyl chloride by substituting HCl for HBr, and reacting it with ethanol but noticed the the rxn rate was sluggish. What is the reason for this decreased reactivity?

A. Chloride is a weaker nucleophile than bromide is
B. Chloride is a weaker electrophile than bromide is
C. Chloride stabilizes the intermediate carbocation that is produced
D. Chloride destabilizes the intermediate carbocation that is produced.

The answer is A

I picked B, because based from periodic trends chloride is a better nucleophile than bromine is and there wasnt any solvent specified in the problem or in the passage. I kno that since this is an sn2 rxn that the rate is dependent on the electrophile (ethanol) and the nucleophile (Cl-). but I dont understand how Cl- can possibly be a worse nucleophile than (Br-).

help!

the question references reaction rate and therefore is targeting our understanding of kinetics. remember that nucleophilicity is a kinetic term pertaining to how good a nucleophile is at seeking out an electrophile. they are thus asking why might the reaction rate be different if we start swapping nucleophiles.

eliminate B first because Cl- is not an electrophile. it is an anion with thus has a lone pair to donate making it a nucleophile and lewis base. next eliminate C because this is contrary to what they are saying (stabilizing the intermediate would lower that activation energy and speed up the reaction, not make it more sluggish--moreover this is what a catalyst would do). answer choice A is the best answer because it explains why the reaction would be slower. if we have a weaker nucleophile then the rate slows down.

the halogens can have different nucleophilic trends depending on the what solvent they are in (protic vs. aprotic--in protic solvents Cl- is a worse nucleophile than Br- because it is smaller and thus more hindered (solvated) by the protic solvent). however, we don't need to know the solvent here to answer the question.

 

[PiBond]

Remember the periodic trends: elements down and to the left have more Nucleophilic behavior: Br is below Cl. Electronegativity reduces nucleophilicity. Nu bind to Carbon, not Hydrogen.

this is incorrect.

also, hydrogen can be an electrophile (ie in an alkene addition of HCl, the hydrogen is the electrophile while the pi bond is the nucleophile)

 

[AcclratngCharge]

this is incorrect. [referring to electronegativity decreasing nucleophilicity]

also, hydrogen can be an electrophile (ie in an alkene addition of HCl, the hydrogen is the electrophile while the pi bond is the nucleophile)

That is wrong. Look it up. Electronegativity is how strongly an atom attracts electrons in a bond. Cl- is very electronegative, it is therefore less willing to share it's electrons, ie. less willing to bond to another atom.

 

[PiBond]

That is wrong. Look it up. Electronegativity is how strongly an atom attracts electrons in a bond. Cl- is very electronegative, it is therefore less willing to share it's electrons, ie. less willing to bond to another atom.

if you go by the phrase "electronegativity reduces nucleophilicity" you will miss every single question comparing halogen nucleophiles in aprotic solvents.

F-> Cl- > Br- > I- (in aprotic)

Also, who's a better nucleophile?...F- or tert-butoxide? Fluorine of course, despite the fact that it's the most electronegative element

my point is: there's a lot that goes into comparing nucleophiles. we can't make blanket statements that "electronegativity decreases nucleophilicity"

 

[AcclratngCharge]

if you go by the phrase "electronegativity reduces nucleophilicity" you will miss every single question comparing halogen nucleophiles in aprotic solvents.

F-> Cl- > Br- > I- (in aprotic)

Also, who's a better nucleophile?...F- or tert-butoxide? Fluorine of course, despite the fact that it's the most electronegative element

my point is: there's a lot that goes into comparing nucleophiles. we can't make blanket statements that "electronegativity decreases nucleophilicity"

The OP's question was concerning anions(Br- vs Cl-), not molecules such as tert-butoxide (electronegativity doesn't describe molecules, it describes atoms).

For monatomic anionic species, electronegativity decreases nucleophilicity; so moving up and to the right on the periodic table decreases nucleophilicity.

 

[thn1401]

How can you even answer this question without knowing if the condition is protic or aprotic?

Since in protic solvent, nucleophilic strength: I>Br>Cl>F
And in aprotic solvent: F>Cl>Br>I

Since the reactant is ethanol, I'm assuming it's protic condition. Br is a better nucleophile than Cl in protic condition because it's harder for larger anion to be solvated. Smaller anion is easier to be solvated and has its electron pair binded by the protic solvent hydrogen and results in weaker nucleophilic.

 

[AcclratngCharge]

Ok, this discussion with ion size, protic or aprotic solvent, it doesn't matter, MCAT isn't that complicated. The MCAT is testing big picture concepts that concern basic rules. I don't know where you got this question, but I'll try to break down this simple question:

In an experiment students tried to synthesize an alkyl chloride by substituting HCl for HBr, and reacting it with ethanol but noticed the the rxn rate was sluggish. What is the reason for this decreased reactivity?

A. Chloride is a weaker nucleophile than bromide is
(this is only one that could be right, POE)
B. Chloride is a weaker electrophile than bromide is
(know that Cl- isn't an electrophile for the MCAT)
C. Chloride stabilizes the intermediate carbocation that is produced
(we know primary substrates [H3C-CH2-OH] undergoes SN2, so there is no carbocation involved)
D. Chloride destabilizes the intermediate carbocation that is produced.
(wrong for same reason as C)

The answer is A

I picked B, because based from periodic trends chloride is a better nucleophile than bromine is and there wasnt any solvent specified in the problem or in the passage. I kno that since this is an sn2 rxn that the rate is dependent on the electrophile (ethanol) and the nucleophile (Cl-). but I dont understand how Cl- can possibly be a worse nucleophile than (Br-).

help!

 

[orgohacks]

if you go by the phrase "electronegativity reduces nucleophilicity" you will miss every single question comparing halogen nucleophiles in aprotic solvents.

F-> Cl- > Br- > I- (in aprotic)

Also, who's a better nucleophile?...F- or tert-butoxide? Fluorine of course, despite the fact that it's the most electronegative element

my point is: there's a lot that goes into comparing nucleophiles. we can't make blanket statements that "electronegativity decreases nucleophilicity"

Exactly. The nucleophilicity of a species can change drastically depending on conditions (e.g. solvent). Note how the question was very clear in pointing out the solvent (protic) - had the solvent been DMSO or acetonitrile, for example, the relative nucleophilicity of Cl- and Br- would have been different.

 

[Chocolatebear89]

F-> Cl- > Br- > I- (in aprotic)

Why does nucleophilicity change in aprotic? Why do they differ? Why is F->Cl- in aprotic?

 

[Rabolisk]

In polar protic solvents, the nucleophile, which is typically an anion, is strongly solvated by the hydrogen bonding solvent. While you do want the entire reaction to occur in one solution, you don't want such a strong solvation, because that blocks the anion from reacting quickly with the electrophile. This effect is biggest for fluorides because they are so small and not easily polarisable. In a polar aprotic solvent, this anion solvation doesn't occur to the same extent, and thus fluoride, which is more reactive than the iodide (think basicity), will be a stronger nucleophile.