Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta
    Offline

    0
    ReputationRep:
    (Original post by Sahir)
    NO! Free radical substitution involves FREE RADICALS, not ELECTROPHILES. A Free radical is any species with a single unpaired electron.. which usually result from homolytic fission; a covalent bond breaks and one of the electrons is received by EACH of the atoms previous bonded by the shared pair. You understand?

    Free radicals are HIGHLY HIGHLY reactive.. they basically nick electrons off anything.. For example in chlorination of an alkane..

    CH4 + Cl* --> CH3* + HCl
    and then CH3* + Cl2 --> CH3Cl + Cl*

    So you see it goes round and round and round.. the chlorine radicals from the second feed back into the first reaction.. hence "propogation".

    Termination is when two radicals bond to one another.. eg..

    CH3* + Cl* --> CH3Cl

    SO, A FREE RADICAL IS NOT AN "ELECTROPHILE".. the name of a mechanism is made up of two parts.. the species in the reaction.. eg. EITHER an electrophile OR a nucleophile OR a free radical.. and then it's either substitution or addition, depending on what that species is doing in the reaction.
    free radicals are nucleophilic eh?
    Offline

    0
    ReputationRep:
    Strictly speaking free radicals can't be classified as either electrophilic or nucleophilic, as while they can act as nucleophiles by attacking a positive centre they can also attack other radicals of the same charge. Radicals are just unstable species formed when a molecule is caused to split up, for example by UV light, and will attack pretty much anything which will rid them of their "extra" electron.
    Offline

    0
    ReputationRep:
    this has nothing to do with the current topic i was doing some revision and wanted to clarify something to mak sure i understood it correctly.
    the way to distinguish between primary and secondary alcohols is by smell.

    when primary alcohol oxidises to aldyhyde - distill smells like rotten apples
    when aldyhde oxidises further carboxylic acid -distill smells like vinegar

    when secondary alcohol oxidises to ketone -distill smells like fruit

    (thats what i interpreted from OCR book)
    is this the only way...i know the acidified potassium dichromate doesent change for tertiary but im asking speficlally for primary and secondary....if so, kind of unfair for people who dont have a sense of smell :rolleyes:

    thank you
    Offline

    2
    ReputationRep:
    (Original post by z!D4N)
    free radicals are nucleophilic eh?
    Bloody hell, a free radical is NEITHER a nucleophile nor an electrophile.. the things will nick electrons off anything they can get their proverbial mitts on!
    Offline

    2
    ReputationRep:
    (Original post by calumc)
    Strictly speaking free radicals can't be classified as either electrophilic or nucleophilic, as while they can act as nucleophiles by attacking a positive centre they can also attack other radicals of the same charge. Radicals are just unstable species formed when a molecule is caused to split up, for example by UV light, and will attack pretty much anything which will rid them of their "extra" electron.
    Here here, sir.
    Offline

    0
    ReputationRep:
    (Original post by z!D4N)
    free radicals are nucleophilic eh?
    No!!!! A species is EITHER electrophilic, nucleophilic OR a free radical a species cannot be both.

    An electrophile has 6 electrons in its outer shell, giving it a positive charge, which is why its attracted to 'electron- rich' areas where it accepts an electron pair.

    A nucleophile has 8 electons in its outer shell, it has a lone pair of electrons which is why it is attracted to areas of positive charge. It donates both of the lone pair of electrons.

    A free radical has 7 electrons in its outer shell making it highly reactive and it will react with pretty much anything to fill its outer shell.
    Offline

    0
    ReputationRep:
    (Original post by shoe_box10)
    this has nothing to do with the current topic i was doing some revision and wanted to clarify something to mak sure i understood it correctly.
    the way to distinguish between primary and secondary alcohols is by smell.

    when primary alcohol oxidises to aldyhyde - distill smells like rotten apples
    when aldyhde oxidises further carboxylic acid -distill smells like vinegar

    when secondary alcohol oxidises to ketone -distill smells like fruit

    (thats what i interpreted from OCR book)
    is this the only way...i know the acidified potassium dichromate doesent change for tertiary but im asking speficlally for primary and secondary....if so, kind of unfair for people who dont have a sense of smell :rolleyes:

    thank you
    You can distinguish between primary and secondary alcohols by refluxing both with excess of acidified potassium dichromate, so the primary alcohol will be oxidised to carboxylic acid.

    Then add Tollen's reagent to both solutions and if a silver mirror forms on the test tube then a carboxylic acid is present as a silver mirror will not form in the presence of a ketone, and therefore the alcohol is primary.

    This is A2 stuff though!
    Offline

    2
    ReputationRep:
    (Original post by shoe_box10)
    this has nothing to do with the current topic i was doing some revision and wanted to clarify something to mak sure i understood it correctly.
    the way to distinguish between primary and secondary alcohols is by smell.

    when primary alcohol oxidises to aldyhyde - distill smells like rotten apples
    when aldyhde oxidises further carboxylic acid -distill smells like vinegar

    when secondary alcohol oxidises to ketone -distill smells like fruit

    (thats what i interpreted from OCR book)
    is this the only way...i know the acidified potassium dichromate doesent change for tertiary but im asking speficlally for primary and secondary....if so, kind of unfair for people who dont have a sense of smell :rolleyes:

    thank you
    Hardly the most obvious way, to be honest!

    If you just follow the oxidation method as you describe, and you find that orange dichromate (VI) reduces to green chromium (III).. you know its either a primary or secondary alcohol. If not, it's tertiary.

    Now repeat the method if it turned green, but distil the product into aq ammoniacal silver nitrate solution (you know might know it as "Tollens").. you either get a silver mirror produced, or not. If there IS a silver mirror, this indicates an aldehyde was produced as a result of oxidation.. the alcohol was primary. If no silver mirror, the product was a ketone and so the alcohol was secondary!

    Sorry i don't do OCR, but you could also distinguish between the aldehyde / ketone using Fehlings (like Benedicts reagent, if u do Biology).. which turns red (Copper (I) Oxide precipitate) in the presence of aldehyde, and no change in presence of ketone. Also need to warm it gently i think.
    Offline

    0
    ReputationRep:
    (Original post by fluffysheep2000)
    No!!!! A species is EITHER electrophilic, nucleophilic OR a free radical a species cannot be both.

    An electrophile has 6 electrons in its outer shell, giving it a positive charge, which is why its attracted to 'electron- rich' areas where it accepts an electron pair.

    A nucleophile has 8 electons in its outer shell, it has a lone pair of electrons which is why it is attracted to areas of positive charge. It donates both of the lone pair of electrons.

    A free radical has 7 electrons in its outer shell making it highly reactive and it will react with pretty much anything to fill its outer shell.
    alright... i understand now... cuz a free radical doesn't have a lone pair of electrons to donate right? so it is not nucleophilic then.
    Offline

    0
    ReputationRep:
    (Original post by z!D4N)
    alright... i understand now... cuz a free radical doesn't have a lone pair of electrons to donate right? so it is not nucleophilic then.
    Exactly for a species to be nucleophilic it must have a lone pair of electrons
    Offline

    0
    ReputationRep:
    (Original post by Sahir)
    Hardly the most obvious way, to be honest!

    If you just follow the oxidation method as you describe, and you find that orange dichromate (VI) reduces to green chromium (III).. you know its either a primary or secondary alcohol. If not, it's tertiary.

    Now repeat the method if it turned green, but distil the product into aq ammoniacal silver nitrate solution (you know might know it as "Tollens").. you either get a silver mirror produced, or not. If there IS a silver mirror, this indicates an aldehyde was produced as a result of oxidation.. the alcohol was primary. If no silver mirror, the product was a ketone and so the alcohol was secondary!

    Sorry i don't do OCR, but you could also distinguish between the aldehyde / ketone using Fehlings (like Benedicts reagent, if u do Biology).. which turns red (Copper (I) Oxide precipitate) in the presence of aldehyde, and no change in presence of ketone. Also need to warm it gently i think.
    One of the few pieces of insight I got from a former Chemistry teacher was that using Benedict's reagent for this purpose was "wrong chemistry", though she never went into detail as to why. It used to annoy her terribly that it was listed as an acceptable answer in the syllabus.
    Offline

    2
    ReputationRep:
    (Original post by calumc)
    One of the few pieces of insight I got from a former Chemistry teacher was that using Benedict's reagent for this purpose was "wrong chemistry", though she never went into detail as to why. It used to annoy her terribly that it was listed as an acceptable answer in the syllabus.
    Huh?! "wrong chemistry" .. what a comedienne.

    It works, heh i've done it in the lab.. i don't care how "wrong" she might reckon it is, cuz u definately get a red ppte
    Offline

    0
    ReputationRep:
    (Original post by Sahir)
    Huh?! "wrong chemistry" .. what a comedienne.

    It works, heh i've done it in the lab.. i don't care how "wrong" she might reckon it is, cuz u definately get a red ppte
    I know, it works, but she didn't like it for some reason. I do remember seeing some kind of reasoning for it, I'll have a look when I'm bored.
    Offline

    2
    ReputationRep:
    hello peeps. anyone here do edexcel?
    im currently revising for topic 2 re-take and am wondering..
    do we need to know stages of free radicall subs, ie propogation etc.. or just the basic idea?
    and also, do we need to know in detail about sp3 and sp2 hybridisation? or just pi and sigma bonds in covalent bonds?
    cheeerss
    Offline

    0
    ReputationRep:
    thank you again everyone for your help, i think ill stick to the Tollen's reagent because it sounds vaguely familiar :rolleyes:
    Offline

    2
    ReputationRep:
    (Original post by misty)
    hello peeps. anyone here do edexcel?
    im currently revising for topic 2 re-take and am wondering..
    do we need to know stages of free radicall subs, ie propogation etc.. or just the basic idea?
    and also, do we need to know in detail about sp3 and sp2 hybridisation? or just pi and sigma bonds in covalent bonds?
    cheeerss

    No, you don't need to know mechanisms for this unit, but I think it would be wise to learn about hybridisation if you don't already know it.
    Offline

    2
    ReputationRep:
    (Original post by Ralfskini)
    No, you don't need to know mechanisms for this unit, but I think it would be wise to learn about hybridisation if you don't already know it.
    k cheers. the syllabus says explain reactivity in terms of diff types of cov bond or something like that.. so it cant be wanting too much detail :confused:
    Offline

    2
    ReputationRep:
    (Original post by misty)
    hello peeps. anyone here do edexcel?
    im currently revising for topic 2 re-take and am wondering..
    do we need to know stages of free radicall subs, ie propogation etc.. or just the basic idea?
    and also, do we need to know in detail about sp3 and sp2 hybridisation? or just pi and sigma bonds in covalent bonds?
    cheeerss
    I did Unit 2 last year, but i'm familiar..

    You don't need to know ANY mechanisms for this unit, so as long as you know the main reaction, for example the chlorination of methane, and the conditions, you'll be fine.

    CH4 + Cl2 --> CH3Cl + HCl ; chloromethane produced
    CH3Cl + Cl2 --> CH2Cl2 + HCl ; dichloromethane produced
    etc etc etc.. this reaction is "messy".. produces loads and loads of different compounds as a result of the mechanism (if you do A2 you'll know about the stages in free radical substitution). Ah yeah, and you need UV light for the homolytic fission of Cl2 --> 2Cl*

    For hybridisation, as long as you vaguely know what's going on.. i.e.: that all of the orbitals around a carbon atom in an alkane or whatever compound are EQUIVALENT.. and hence they're all sp3 hybridised.. tetrahedral shape and a non-polar compound as all bond dipoles cancel. So essentially yeah, know that alkanes = 4 x sp3 hybridised orbitals, and alkenes = 3 x sp2 hybridised + 1 p-orbital.. overlapping adjacent orbitals gives pi bond perpendicular to the plane.

    For Pi / Sigma bonds, know a definition for each, that's all really.

    Pi = molecular orbital formed as a result of the lateral overlap of electron density in p-orbitals of adjacent atoms. Perpendicular to plane of sigma-bonds.. hence why you get geometric isomers of alkenes and electrophilic addition due to exposed electron density.

    Sigma = molecular orbital formed as a result of the side-side overlap of electron density in s-orbitals (tecnically they're hybridised) of adjacent atoms.
    Offline

    2
    ReputationRep:
    (Original post by Sahir)
    I did Unit 2 last year, but i'm familiar..

    You don't need to know ANY mechanisms for this unit, so as long as you know the main reaction, for example the chlorination of methane, and the conditions, you'll be fine.

    CH4 + Cl2 --> CH3Cl + HCl ; chloromethane produced
    CH3Cl + Cl2 --> CH2Cl2 + HCl ; dichloromethane produced
    etc etc etc.. this reaction is "messy".. produces loads and loads of different compounds as a result of the mechanism (if you do A2 you'll know about the stages in free radical substitution). Ah yeah, and you need UV light for the homolytic fission of Cl2 --> 2Cl*

    For hybridisation, as long as you vaguely know what's going on.. i.e.: that all of the orbitals around a carbon atom in an alkane or whatever compound are EQUIVALENT.. and hence they're all sp3 hybridised.. tetrahedral shape and a non-polar compound as all bond dipoles cancel. So essentially yeah, know that alkanes = 4 x sp3 hybridised orbitals, and alkenes = 3 x sp2 hybridised + 1 p-orbital.. overlapping adjacent orbitals gives pi bond perpendicular to the plane.

    For Pi / Sigma bonds, know a definition for each, that's all really.

    Pi = molecular orbital formed as a result of the lateral overlap of electron density in p-orbitals of adjacent atoms. Perpendicular to plane of sigma-bonds.. hence why you get geometric isomers of alkenes and electrophilic addition due to exposed electron density.

    Sigma = molecular orbital formed as a result of the side-side overlap of electron density in s-orbitals (tecnically they're hybridised) of adjacent atoms.

    These are all good definitions but you need to appreciate that sigma and pi bonds aren't exclusive to s and p-orbitals respectively, so it is not advisable to include this in a definition of the terms.
    Offline

    2
    ReputationRep:
    (Original post by Ralfskini)
    These are all good definitions but you need to appreciate that sigma and pi bonds aren't exclusive to s and p-orbitals respectively, so it is not advisable to include this in a definition of the terms.
    In terms of AS knowledge, it is. I just checked in a unit 2 mark scheme.. particularly for pi-bonds, this is.
 
 
 
Poll
Do you agree with the proposed ban on plastic straws and cotton buds?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.