Homework questions thread

[charmanderp]

My understanding is that the only question you'll get to do with the IR process is something like "Which of the following is part of the method for IR" with correct options like "Covalent bonds stretch due to absorption of energy". So I wouldn't worry too much about that.

[thushan]

My understanding is that the only question you'll get to do with the IR process is something like "Which of the following is part of the method for IR" with correct options like "Covalent bonds stretch due to absorption of energy". So I wouldn't worry too much about that.

There's another type of Q you can get. Check VCAA 2011 Q 18 Multichoice.

[Mr. Study]

Hey all.

I just need a little help with his IR spectrum.

Would we consider the little peak at approximately 2750-2900?

If not, Could you be able to explain why? Should we only consider peaks that are quite prominent, like the one shown at ~1750?

Thanks!

[thushan]

Hey all.

I just need a little help with his IR spectrum.

Would we consider the little peak at approximately 2750-2900?

If not, Could you be able to explain why? Should we only consider peaks that are quite prominent, like the one shown at ~1750?

Thanks!

That looks like a C-H stretch. A C-H stretch is a very sharp peak produced at around 2800-3000 cm-1. Basically every organic compound has a C-H bond, so there will always be an absorption band there.

[soccerboi]

Could i name this as 2-methyl-2-bromopropane instead of 2-bromo-2-methylpropane?

[jadams]

Could i name this as 2-methyl-2-bromopropane instead of 2-bromo-2-methylpropane?

Nope, it's alphabetical when it comes to naming side groups. ie bromo comes before chloro, chloro would come before methyl etc....

[charmanderp]

And also note that prefixes for prefixes (such as di, tri, etc) don't make any difference to naming ie. dibromo would still come before chloro, and trichloro would still come before methyl.

[charmanderp]

The fatty acids are carboxylic acids with really long carbon chains. The saturated fatty acids do not possess any double bonds within this carbon chain (no C=C double bond, although there is a C=O double bond at the end) and hence possess the maximum number of hydrogen atoms possible.

Glycerol is an alcohol, and we know that when alcohols are mixed with carboxylic acids, they form esters. Since glycerol has three O-H groups, each molecule can engage in condensation reactions with three fatty acids, to form what is known as a triglyceride.

Since we're here using saturated fatty acids (which have the general formula CnH2n+1COOH), we get saturated fats. These are solids at room temperature, as the fatty acids are able to align themselves really closely too each other, due to being straight chains with no double bonds). Unsaturated fats are oils are room temperature, as the double bonds weakens the solidarity of the overall molecule, as it can't pack itself tightly in.

Unlike the unsaturated fatty acids, you can't hydrogenate a saturated fatty acids, or saturated fat.

I feel like I've forgotten something.

[sin0001]

How are biodiesels formed, what's reacted with alcohol molecules? Can someone differentiate between biochemical fuels and biodiesel

[ggxoxo]

Biodiesel is a type of biochemical fuel. The 3 biochemical fuels you must know are: bioethanol, biodiesel and biogas.

Biodiesel is usually made from a triglyceride + alcohol (e.g. methanol) in a process that is usually base-catalysed transesterification.

The general eqn is:

triglyceride+ alcohol (e.g. ethanol) -->(catalyst-NaOH) 3 R-COOR' (biodiesel- 'methyl ester') + glycerol

Sometimes the qn might just ask you to react a 1 fatty acid that has already been hydrolysed (instead of a whole triglyceride), in that case no glycerol is made and only 1 biodisel is formed.

For example, palmitic acid + methanol

C15H31COOH + CH3OH --> C15H31COOCH3 +water

As you can probably see from both examples, forming biodiesel is just creating an ester bond (hence, the name 'tranesterification').

[soccerboi]

How do you know what states hydrocarbons are in? I know that it is provided in the data booklet but those are for combustion reactions. How about other reactions, would these states still apply?

[thushan]

How do you know what states hydrocarbons are in? I know that it is provided in the data booklet but those are for combustion reactions. How about other reactions, would these states still apply?

not just combustion reactions - those states are for hydrocarbons at room temperature, so you can use the ones in the data booklet.

[soccerboi]

How do you know what states hydrocarbons are in? I know that it is provided in the data booklet but those are for combustion reactions. How about other reactions, would these states still apply?

not just combustion reactions - those states are for hydrocarbons at room temperature, so you can use the ones in the data booklet.

Oh ok, but how come the data booklet says ethanol is liquid, but when you derive ethanol from fermentation of glucose it's in aq state?

[BlueSky_3]

aq=aqueous....which just means a liquid dissolved in water.

[soccerboi]

aq=aqueous....which just means a liquid dissolved in water.

yeh so either would be fine right? no marks lost?