test for acyl chlorides

Original post by Bertybassett

how do you do q2 on this paper. Surely theres no chloride ions in solution so how would a white precipitate be formed with agno3? can someone please explain? thanks

https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Topic-Qs/AQA/Organic-II/3.09-Carboxylic-Acids-and-Derivatives/Set-F/Acylation%E2%80%8B%202%20QP.pdf

What reactions of acyl chlorides do you know?

What is present in your test solution?

Reply 2

OP

I know the reactions with water, alcohol, amines, and primary amines. What I am confused is how cl- ions become present? For example, when testing for 1 bromo propane, I never understand when you do and dont add naoh before adding agno3?

Original post by Pigster

What reactions of acyl chlorides do you know?

What is present in your test solution?

Reply 3

Kian Stevens

16

Original post by Bertybassett

I know the reactions with water, alcohol, amines, and primary amines. What I am confused is how cl- ions become present? For example, when testing for 1 bromo propane, I never understand when you do and dont add naoh before adding agno3?

You always add aqueous NaOH to haloalkanes when testing for them, as this produces halide salt

As for acid halides, you also always add aqueous NaOH to them, to base hydrolyse them. This occurs in two steps:

1.

Hydrolysis of acid halide to produce carboxylic acid and hydrogen halide

2.

Acid-base reaction of hydrogen halide and NaOH to produce halide salt (as well as water and sodium carboxylate salt but that's irrelevant here)

Hence, in both cases, the halide salt reacts with the AgNO₃ to produce the silver halide precipitate

(edited 4 years ago)

Reply 4

Original post by Kian Stevens

You always add aqueous NaOH to haloalkanes when testing for them, as this produces halide salt

As for acid halides, you also always add aqueous NaOH to them, to base hydrolyse them. This occurs in two steps:

1.

Hydrolysis of acid halide to produce carboxylic acid and hydrogen halide

2.

Acid-base reaction of hydrogen halide and NaOH to produce halide salt (as well as water and sodium carboxylate salt but that's irrelevant here)

Hence, in both cases, the halide salt reacts with the AgNO₃ to produce the silver halide precipitate

The problem with your suggestion is that the 'halide salt' you make is mixed with the excess NaOH.

NaOH reacts with AgNO3 producing a lovely brown sludge that will hide your white ppt.

Reply 5

Kian Stevens

16

Original post by Pigster

The problem with your suggestion is that the 'halide salt' you make is mixed with the excess NaOH.

NaOH reacts with AgNO3 producing a lovely brown sludge that will hide your white ppt.

Meh never even considered that

Reply 6

JanaALEVEL

10

Can you link the MS please ?

Reply 7

Original post by Bertybassett

how do you do q2 on this paper. Surely theres no chloride ions in solution so how would a white precipitate be formed with agno3? can someone please explain? thanks

https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Topic-Qs/AQA/Organic-II/3.09-Carboxylic-Acids-and-Derivatives/Set-F/Acylation%E2%80%8B%202%20QP.pdf

It is a question about rates. Both make the white ppt, one does it instantly, though.

Reply 8

OP

ok so just to clarify, with acyl chlorides, adding agno3 will produce a white precipitate instantly but with haloalkanes it is produced more slowly.
When testing for haloakanes, you add naoh first always, then add acidified agno3?

Original post by Pigster

It is a question about rates. Both make the white ppt, one does it instantly, though.

Reply 9

OP

so do you always add naoh first? like some questions say add naoh first, others dont. im so confused

Original post by Kian Stevens

Meh never even considered that

Reply 10

Original post by Bertybassett

so do you always add naoh first? like some questions say add naoh first, others dont. im so confused

There is no mention of NaOH in Q2.

The water in aqueous silver chloride will hydrolyse the haloalkane, rather slowly releasing Cl-.

The water in aqueous silver chloride will hydrolyse the acyl chloride, instantly releasing Cl-.

Reply 11

Kian Stevens

16

Original post by Pigster

There is no mention of NaOH in Q2.

The water in aqueous silver chloride will hydrolyse the haloalkane, rather slowly releasing Cl-.

The water in aqueous silver chloride will hydrolyse the acyl chloride, instantly releasing Cl-.

...then why has this entire discussion been about NaOH?

Reply 12

… beats me.

Original post by Kian Stevens

...then why has this entire discussion been about NaOH?

Reply 13

OP

I know there wasn't. im just asking a side question because its something that has confused me for a long time, i've already made 2 threads literally asking about naoh and getting different responses. So when do we add naoh first when testing for haloalkanes?

Original post by Pigster

There is no mention of NaOH in Q2.

The water in aqueous silver chloride will hydrolyse the haloalkane, rather slowly releasing Cl-.

The water in aqueous silver chloride will hydrolyse the acyl chloride, instantly releasing Cl-.

Reply 14

Original post by Bertybassett

I know there wasn't. im just asking a side question because its something that has confused me for a long time, i've already made 2 threads literally asking about naoh and getting different responses. So when do we add naoh first when testing for haloalkanes?

It depends on whether you are in a hurry and/or whether you're trying to compare rate of hydrolysis between the various haloalkanes.

You can add NaOH for a nice fast hydrolysis, then acidify to remove the OH- and then add AgNO3 to make the coloured ppt.

But, if you are trying to compare rates, you'd just use water as the nucleophile (rather than OH-) and start with the Ag+, then see how long it takes for a cross to be obscured, c.f. thio + acid.

Reply 15

OP

ok thanks. so adding acidifified agno3 to say bromoethane will result in a cream precipitate, but will just take a while to develop. But you could also add naoh then hno3, the agno3 for a faster formation of the precipitate?

Original post by Pigster

It depends on whether you are in a hurry and/or whether you're trying to compare rate of hydrolysis between the various haloalkanes.

You can add NaOH for a nice fast hydrolysis, then acidify to remove the OH- and then add AgNO3 to make the coloured ppt.

But, if you are trying to compare rates, you'd just use water as the nucleophile (rather than OH-) and start with the Ag+, then see how long it takes for a cross to be obscured, c.f. thio + acid.

Reply 16

Original post by Bertybassett

But you could also add naoh then hno3, the agno3 for a faster formation of the precipitate?

If you add just NaOH, then hydrolysis will take place, at a faster rate than just adding AgNO3(aq), but no ppt forms, so you can't tell anything about the rate.

Reply 17

OP

what do you mean no precipitate forms? in mark schemes ive seen them say add naoh first then look for white precipitate etc? honestly im so confused, been trying to find the answer to this for ages and still dont know. do I or do I not add naoh first in the test for halides (ignoring rates)?

Original post by Pigster

If you add just NaOH, then hydrolysis will take place, at a faster rate than just adding AgNO3(aq), but no ppt forms, so you can't tell anything about the rate.

Reply 18