1.7 Introductory organic chemistry
Posted by: intechemistry on: September 23, 2010
Hazard and risk. three links to read (in order of increasing length):
1) http://www.worksmart.org.uk/health/
what_is_the_difference_between_hazard_and
2) www.dehp-facts.com/CLab/CL_hazard.htm
3) http://www.agius.com/hew/resource/hazard.htm
11 Responses to "1.7 Introductory organic chemistry"
December 11, 2010 at 1:34 am
Salam. It’s best to use bromine water. Br2(aq)
Imagine we decided to use just bromine. Bromine is liquid at RTP. A pure liquid. The density is about 3.1 g·cm^−3 so using 1cm^3 of Br2, you have 0.0125 moles. To make all that bromine go colourless you have to bubble something like 300cm3 of ethene gas. That’s a lot of gas and we only used 1cm^3 of bromine!!
If we have bomine water (we usually have something a 0.1M solution, meaning 0.1 moles in 1000cm^3 so in 1cm^3 there is at most, 0.0001 moles of Br2 only). That needs only about 2.4cm^3 of ethese gas to decolourise. 
Hopefully now you can appreciate why bromine water is better, but you could still use pure bromine – but you would need to pass a LOT of ethene through it. Not such a good thing just to do a chemical test, yes?
Br2(l) + alkene does indeed go from red-brown to colourless (on addition of sufficient alkene)
Br2(aq) + alkene goes from orange-brown coloured solution to colourless.
Note: (1 mole of gas at RTP accupies about 24,000 cm^3, i.e. a cube of dimensions 28x28x28cm)
December 19, 2010 at 8:25 am
salam sir..
i have a confusion here regarding electrophilic addition mechanism.
in page 132 & 133 AS, there is a contradiction in the two pages. in pg 132, it says that the first stage is electrophilic attack, followed by neucleophilic attack, but on page 133, it is vice versa. what i understand here is it is the alkene electron rich pai bond will approach the other molecule first, & threfore induce it to become positive, then only it will be attacked by the electrophile. so which one first actually? electrophilic attack or neuclophilic attack?
and when i go thru my notes, it says that if ekectrophilic addition, then it is the positive particles which attack the electron dense region…
*_*(confused)
btw, happy weekend sir…
December 23, 2010 at 3:36 pm
Salam and sorry Salha, I’ve not been online for a while.
I only have the Pearson book, but its not with me at the moment, so I guess I don’t really have it after all and (I lent out Hodder and Facer. Which book are you referring to?
You are right, alkenes contain an electron rich area in the pi bond. It is these electrons whch attach electrophiles. In mechanisms we draw the curly arrow from the pi bond pointing towards the electrophile. This is called electrophilic attack (attack occurs and it involves an electrophile).
I think I need to look at the book before I can understand why they do as you described. I’m not sure when I’ll get those books back so perhaps you could scan the pages and e-mail them to me – upon receipt I’ll check them out. If agreeable, I’ll sms you my e-mail.
December 24, 2010 at 1:17 am
Salha. I see it in Pearson. I understand the problem.
On p132, it’s talking about the H of H-Br attacling the C=C hence electrophilic attack (by H-Br on C=C) but in page 133, it now adopts the perspective in words(!)of the C=C attacking the Br2. Hence the C=C acting as a nucleophile. What they say is actually valid but not very helpful!
If our point of reference changes then so will the description. Sitting on a smooth train at constant velocity, the train seems still and the rest of the world is moving. Standing on the ground, the train in moving and the world stay still.
Actually both reactions [alkenes with HBr and Br2] are usually called electrophilic addition as the organic molecule is usually taken as ‘fixed’ and how the other species reacts with it is used to donate the type of reaction:
OH-(aq) attacking a R-X is a nucleophilic substitution. :CN- on carbonyls is a nucleophilic addition. NO2+ on benxene is an electrophilic addition. The OH-(aq), :CN- and NO2+ are a nucleophile, nuleophile again, then finally electrophile.
obviously if what they are doing isn’t explicitily stated.
My guess as to why ‘Person’ does this is:
In the HBr example it’s the delta+ H (i.e. the non-organic molecule) which causes the pi electrons to move towards the H, but with Br-Br, it’s the C=C (the organic molecule) that causes increased polariasion of Br2 which then leads to attack. If that was their thinking, I think it’s kind of a weak way of doing it which can be very much improved.
December 28, 2010 at 9:27 pm
salam mr allan..
thank you very much for your explanation…i’m sorry for taking like ages to reply your comment..i was taking some time to go through the notes again n your comment as well…and alhamdulillah.. glad that i understand already(sorry, i’m a slow learner)…however..thnx again..i’m much obliged..^_^
January 18, 2011 at 1:00 am
salam sir.
1. why trans-isomer is more harmful than cis-isomer?
(referring to trans-fatty acids which could cause heart disease, but not cis-fatty acids)
2. in case of vision in human beings; why as the molecule rotates, it reforms as the trans-isomer?
January 18, 2011 at 6:58 am
1.The explanation involves packing efficiency. Trans isomers are more linear and so can pack together more readily compared to the ‘u’ shaped cis isomers. Greater packing ability means greater chnace fatty substances can accumulate and deposit themselves in or block arteries in the body.
2. I don’t know the accepted answer (there may be some enzyme/or ‘assistant’ molecule at work forcing it to adopt that shape) but at first guess, I’d say in the environment it’s in, the internal energy of the trans isomer is less (hence more stable) than it’s cis form.
May 10, 2011 at 12:37 pm
hi sir!
[Chemistry Unit 1 (14 Jan 2010)]
Objective part, Q13:
Which of the following mixtures could NOT form when octane C8H18 is cracked?
A propane + pentene
B butane + butene
C pentane + propene
D heptane + ethene
The answer is D. Why is it so since the equations seem balanced to give 8C and 18H in all reactions A to D above?
May 10, 2011 at 12:52 pm
Because D doesn’t balance.
C7H16 (heptane) + C2H6 (ethene) = C9H22
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December 10, 2010 at 11:28 pm
salam
mr allan, we usually use the electrophilic addition of alkene (let’s say ethene) with halogen compound as a test for alkene.
let’s say we use bromine and ethene.
do we write bromine water or pure liquid bromine because both reactions will result in different products right?
and for the observation, if we write pure liquid bromine, the colour changes would be from red-brown to colourless. am I right?
and if we write bromine water, what would be the observation?