[chemistry] Organic and other chem help

[Mahuta ♥]

I haven't seen a thread about organic chemistry yet, so I'm starting one since I kind of need help.

I would like to know certain things:

1) Does non-cyclical geometric isomerism occur only in alkenes?

2)It says in the study guide that some of the characteristics of geometric isomers are:

- In the cis- isomer, H-bonds could form within the molecule it self.

- in the trans- isomer, H-bond occurs between molecules.

What I dont understand: wouldnt H-bonds form only when you have an OH group on the attached R-groups?

So, how would that characteristic be valid for all cis and trans isomers?

2) I always don't quite understand elimination reactions. I know the mechanism and stuff but the general definition and stuff, I'm not sure of.]

3) For the reaction pathways, the syllabus gives an example of:

1-bromopropane----------------------> 1-butylamine can be done in 2 stages:

a) 1-bromopropane + KCN -------------> propane nitrile

b) propane nitrile --------------------> 1-butylamine

The thing I don't understand is, that when you react 1-bromopropane with CN, shouldn't the carbon chain increase by one? So, why is it still propane?

And if thats the right thing, how can propane nitrile be reduced into butylamine? Where does the 4th carbon come from?(but-)

Thanks in advance.

[moneyfaery]

1. Optical isomerism occurs whenever there are 4 different functional groups. Cis/trans (geometric) isomerism generally only occurs in alkenes because they have a double bond. It can also occur in carboxylic acids, e.g. 1,4-cis-2-butenedioic acid, provided there is a double bond somewhere.

2. I'm not sure about the H-bonding... since there aren't O, F, Cl, or N atoms present in alkenes, H-bonding technically wouldn't exist. However, because cis-molecules are "circular" (arc-shaped) and trans-molecules are "linear" (more or less diagonal), trans-molecules can pack closer together. They also tend to show more van der Waals' ("intermolecular") while cis-molecules show dipole:dipole ("intramolecular").

Edit: See above for 1,4-cis-2-butenedioic acid. H-bonding would exist in this case because of the OH functional groups, so yes, you're right.

3. Elimination reaction is basically when 2 substituents are removed from a molecule, e.g. removing H + Br atoms from bromoethane (saturated) and converting it to ethene (unsaturated). Conditions: heated in alcoholic NaOH solution.

4. a) CN^- replaces the Br halogeno functional group in 1-bromopropane via an S_N² mechanism (activated complex formed).

CBrH₂-CH₂-CH₃ + CN^- --> CH₂CN-CH₂-CH₃ + Br^-

b) When 2H₂ is added with Ni as catalyst, the C from CN becomes part of the carbon chain so propanenitrile --> 1-butanamine

CH₂CN-CH₂-CH₃ (+ H₂ + Ni catalyst) --> CH₃CH₂CH₂CH₂NH₂

The naming is slightly different but it's organic chem, so that's to be expected. I think it's mostly right but I might have made a mistake somewhere.

Edited April 2, 2009 by Irene

[Mahuta ♥]

Ok, another problem, optical isomerism anyone?

I know about the mirror images, but whats 'plane-polarized'?

Also, the chemical and physical properties of enantiomers.

[Sandwich]

Plane polarised light is basically light which has been passed through a vertical slit, or something similar, to put it all into the same plane (the vertical plane). Basically all of the light will be moving through the same plane (thinking of light as a transverse wave).

In terms of optical isomers, different isomers will cause the plane of the light to alter in different directions, by a certain number of degrees. A racemic mixture is where you have an equal mixture of two optical isomers-- one might push light 33 degrees to the left (laevorotatory), the other pushes it 33 degrees to the right (dextreorotarory), so racemic mixtures end up giving light which doesn't appear to have had its plane altered at all. Cancels itself out :]

Chemical and physical properties of enantiomers ought to be identical-- they're pretty much the same molecules except for the whole mirror image thing. The only difference comes when an optical isomer interacts with another molecule which is optically active-- which is mostly in the body. One good example to give is Thalidomide. One enantiomer messes up the growth of babies in the womb, and the other relieves morning sickness.

I hope that's what you wanted to know!

EDIT: I should add that if your textbook has all of the options in it, you may find a slightly fuller explanation of how it all works in Option D, as it's part of the Drugs & Medicines topic too -- in my textbook I found the explanation there easier to understand

Edited April 2, 2009 by sandwich

[Mahuta ♥]

Yeah thanks alot alice, so basically the physical should be the same because its the same structure, fair enough.

Are there any differences in terms of the bonds formed between molecules? For the chemical properties that is.

[Sandwich]

Nope, there shouldn't be! I dunno if you have one of those strange chemistry modelling kits made out of plastic, but if you want to really get the gist of it, try making two versions of the same molecule with a chiral carbon at the centre. You won't be able to stick them on top of each other, but they're perfectly identical in absolutely every other respect ^___^

Drawing it is a bit harder because you can't appreciate the distinctions unless you're particularly adept at imagining the molecule in 3D off the page.

It's all about how it interacts with other molecules. One good way to think of it is that molecules can be either left handed or right handed. Hands are exactly the same between left and right (if you hold your left hand up to a mirror, you see the shape of your right hand reflected) but you can't overlay them on each other. That's literally the only difference between hands, and similarly the only difference between the chiral molecules. In terms of what they can do, think of it like this: your right hand can operate a right-handed pair of scissors where your left hand can't (well it can, but with difficulty, so we'll say it can't). So 'right handed' molecules (i.e. one particular enantiomer) can do right-handed things, but left-handed molecules can't.

That may be over-explained, but the general gist is that you should think of them as perfectly identical except for what they can do interacting with other optically active things (I dunno if you knew this, but glucose is actually a right handed molecule in its useful form, as it has 4 chiral carbons, which I thought was kind-of cool given how important it is). And altering plane-polarised light, of course

[moneyfaery]

Some right/left-handed molecules are toxic... I will elaborate later.

[Sandwich]

For the Further Organic Chem option, p385 of the Course Companion, diagrams 26 and 27, is there any way anybody could possibly explain to me exactly what those diagrams are trying to show i.e. what's actually going on with the directional groups?

Can I pretend this is a generic chem help thread?

With Acid-Base titration curves, it details on the syllabus that we need to know the positions where pKa = pH or pKb = pOH. Is that at the equivalence points?

Edited May 13, 2009 by Mahuta
OK,lol.

[sweetnsimple786]

Using the Henderson-Hasselbalch equation, you get pH= pKa + log ( [base]/[acid] )

when [base]=[acid], pH=pKa, so yes--pH=pKa at the equivalence point.

[Sandwich]

Using the Henderson-Hasselbalch equation, you get pH= pKa + log ( [base]/[acid] )

when [base]=[acid], pH=pKa, so yes--pH=pKa at the equivalence point.

Many thanks

What is the concentration of OH– ions (in mol dm–3) in an aqueous solution in which

[H+] = 2.0×10–3 mol dm–3? (Kw = 1.0×10–14 mol2 dm–6)

A. 2.0×10–3

B. 4.0×10–6

C. 5.0×10–12

D. 2.0×10–17

^^ help with this question?

Apparently the answer is C, but I don't see how to work this out without a calculator and calculating pHs, which of course aren't allowed in multi choice (annoyingly!). I know the general principle that 1.0x10-a + 1.0x10-b will always give a+b=14. However, I don't know how to adjust this general principle for things which don't start 1.0!

[ozolins]

Many thanks

What is the concentration of OH– ions (in mol dm–3) in an aqueous solution in which

[H+] = 2.0×10–3 mol dm–3? (Kw = 1.0×10–14 mol2 dm–6)

A. 2.0×10–3

B. 4.0×10–6

C. 5.0×10–12

D. 2.0×10–17

^^ help with this question?

Apparently the answer is C, but I don't see how to work this out without a calculator and calculating pHs, which of course aren't allowed in multi choice (annoyingly!). I know the general principle that 1.0x10-a + 1.0x10-b will always give a+b=14. However, I don't know how to adjust this general principle for things which don't start 1.0!

We know that Kw = [H+]×[OH-], so solving for [OH-] = Kw/[H+]

inserting the values we have: [OH-]=1.0×10^-14 / 2.0×10^-3 = 0.5×10^(-14+3) = 0.5×10^-11 = 5×10^-12

[Sandwich]

Still with the engaging activity of doing multi choice questions

Which species will show optical activity?

A. 1-chloropentane

B. 3-chloropentane

C. 1-chloro-2-methylpentane

D. 2-chloro-2-methylpentane

Apparently it's C, but I only see a chiral carbon in D.

On the carbon for © you have CH3, Cl, CH3 and C3H7, which gives two methyl groups.

On the carbon for (D), however, there's CH2Cl, CH3, H and C3H7 as the groups.

I don't know whether it's a mistake in the multiple choice or in my reckoning. Any input?

[Tilia]

But what is the definition of the equivalence point? My teacher said it was where the slope changes direction, but I don't really see that...

Also, Alice, is this stuff options or the core? Just interested, it looks difficult...

[sweetnsimple786]

The equivalence point occurs when the number of moles of acid equals the number of moles of base.

you can see it on a graph if you pinpoint the two locations that the curve starts shifting. If you have a strong-strong, then you have that part of the titration curve that looks like a steep line. in the middle of the line (vertically) is where the point is. This also works for strong-weak, weak-strong, weak-weak.

Dang..optical activity. I just crammed for that for the SL Physics exam, option A, so I could tell you how it relates to polarization and LCDs, but I can't draw C and D to tell which has the chiral carbon.

[Sandwich]

But what is the definition of the equivalence point? My teacher said it was where the slope changes direction, but I don't really see that...

Also, Alice, is this stuff options or the core? Just interested, it looks difficult...

Core, as I recall. The equivalence point stuff is Acids & Bases, the chiral carbon stuff is Organic Chem. The directional groups stuff was Further Organic.

The equivalence point is where [HA] = [A-] for a weak acid. If you imagine somebody titrating an acids and a base in a beaker using an indicator, it's the point at which the change of colour occurs. Basically on your average titration curve showing amount added along the x axis and the pH along the y axis, it's the mid-point of the part of the curve where the line goes straight up. Say you're titrating a strong acid with a strong alklali. The solution starts off at an acidic pH, gradually becoming more alkaline as more of the alkali is added... then suddenly when enough alkali has been added, the pH changes very rapidly across a very small range. You need only add one drop of an alkali to make the solution significantly more alkaline. It's mid-way through this sudden rapid change in pH from very acid to very alkaline (for a strong & a weak acid/base, roughly pH 7) that the equivalence point occurs.

More or less it ends up looking like an S shaped curved... if the S had a very straight back!

http://www.chem.fsu.edu/chemlab/chm3120l/acid/sasb.gif

[moneyfaery]

The equivalence point is where [HA] = [A-] for a weak acid. If you imagine somebody titrating an acids and a base in a beaker using an indicator, it's the point at which the change of colour occurs. Basically on your average titration curve showing amount added along the x axis and the pH along the y axis, it's the mid-point of the part of the curve where the line goes straight up. Say you're titrating a strong acid with a strong alklali. The solution starts off at an acidic pH, gradually becoming more alkaline as more of the alkali is added... then suddenly when enough alkali has been added, the pH changes very rapidly across a very small range. You need only add one drop of an alkali to make the solution significantly more alkaline. It's mid-way through this sudden rapid change in pH from very acid to very alkaline (for a strong & a weak acid/base, roughly pH 7) that the equivalence point occurs.

To simplify: it's the midpoint of the 'jump' when the solution goes from being acidic to basic or vice versa. It tells you about the strength of the titrants/titrands used so it could be useful, I guess.

[sweetnsimple786]

The equivalence point is where [HA] = [A-] for a weak acid.

Is it when the concentrations are equal or when the number of moles are equal? For a titration, you'd have an equal volume for the base and acid and a set molarity for each, right?

[Qwaps]

Still with the engaging activity of doing multi choice questions

Which species will show optical activity?

A. 1-chloropentane

B. 3-chloropentane

C. 1-chloro-2-methylpentane

D. 2-chloro-2-methylpentane

Apparently it's C, but I only see a chiral carbon in D.

On the carbon for © you have CH3, Cl, CH3 and C3H7, which gives two methyl groups.

On the carbon for (D), however, there's CH2Cl, CH3, H and C3H7 as the groups.

I don't know whether it's a mistake in the multiple choice or in my reckoning. Any input?

You are mixing up C and D.

[Tilia]

But what is the definition of the equivalence point? My teacher said it was where the slope changes direction, but I don't really see that...

Also, Alice, is this stuff options or the core? Just interested, it looks difficult...

Core, as I recall. The equivalence point stuff is Acids & Bases, the chiral carbon stuff is Organic Chem. The directional groups stuff was Further Organic.

The equivalence point is where [HA] = [A-] for a weak acid. If you imagine somebody titrating an acids and a base in a beaker using an indicator, it's the point at which the change of colour occurs. Basically on your average titration curve showing amount added along the x axis and the pH along the y axis, it's the mid-point of the part of the curve where the line goes straight up. Say you're titrating a strong acid with a strong alklali. The solution starts off at an acidic pH, gradually becoming more alkaline as more of the alkali is added... then suddenly when enough alkali has been added, the pH changes very rapidly across a very small range. You need only add one drop of an alkali to make the solution significantly more alkaline. It's mid-way through this sudden rapid change in pH from very acid to very alkaline (for a strong & a weak acid/base, roughly pH 7) that the equivalence point occurs.

More or less it ends up looking like an S shaped curved... if the S had a very straight back!

http://www.chem.fsu.edu/chemlab/chm3120l/acid/sasb.gif

Right. Just hope we don't have to define it

[Tilia]

Core, as I recall. The equivalence point stuff is Acids & Bases, the chiral carbon stuff is Organic Chem. The directional groups stuff was Further Organic.

The equivalence point is where [HA] = [A-] for a weak acid. If you imagine somebody titrating an acids and a base in a beaker using an indicator, it's the point at which the change of colour occurs. Basically on your average titration curve showing amount added along the x axis and the pH along the y axis, it's the mid-point of the part of the curve where the line goes straight up. Say you're titrating a strong acid with a strong alklali. The solution starts off at an acidic pH, gradually becoming more alkaline as more of the alkali is added... then suddenly when enough alkali has been added, the pH changes very rapidly across a very small range. You need only add one drop of an alkali to make the solution significantly more alkaline. It's mid-way through this sudden rapid change in pH from very acid to very alkaline (for a strong & a weak acid/base, roughly pH 7) that the equivalence point occurs.

OK, thanks. I just hope we don't have to give a definition on a test.