Original post by Kev.1995
Why is it 120 in benzene but 109.5 in cyclohexane?
Which part of AQA Chem is this?
Which part of AQA Chem is this?
Due to electron pair repulsion. Chem 2, shopes and molecules
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Thanks for your reply 
but there are no lone pairs in benzene, only a delocalised ring?
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but there are no lone pairs in benzene, only a delocalised ring?Posted from TSR Mobile
Original post by Kev.1995
Thanks for your reply but there are no lone pairs in benzene, only a delocalised ring?
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but there are no lone pairs in benzene, only a delocalised ring?Posted from TSR Mobile
The carbon atoms in benzene are sp2 hybridised, whereas those in cyclohexene are sp3
Original post by illusionz
x
Hmm, so are the bond angles in any cyclic molecule still tetrahedral (if it's Carbon)? e.g. Butane - bond angle of 109.5 rather than 90? Pentane - bond angle of 109.5 rather than 108? C12H24 - bond angle of 109.5 rather than 150? I thought the shape of the cycle defined the bond angle ...
Original post by Kev.1995
I dont understand why they aren't both 120.. surely it's a fixed ring?
In cyclohexane, each carbon atom is bonded to four other atoms, (two carbon, two hydrogen) this makes it a tetrahedral structure if you think of the shape of the bonding around each carbon atom. Hence, the bond angles is 109.5o.
In benzene, each carbon atom is bonded to three others atoms, (two carbon, one hydrogen) this makes it a trigonal planar structure in terms of the bond angles around each carbon atom, making the bond angles 120o.
Original post by thegodofgod
In cyclohexane, each carbon atom is bonded to four other atoms, (two carbon, two hydrogen) this makes it a tetrahedral structure if you think of the shape of the bonding around each carbon atom. Hence, the bond angles is 109.5o.
In benzene, each carbon atom is bonded to three others atoms, (two carbon, one hydrogen) this makes it a trigonal planar structure in terms of the bond angles around each carbon atom, making the bond angles 120o.
In benzene, each carbon atom is bonded to three others atoms, (two carbon, one hydrogen) this makes it a trigonal planar structure in terms of the bond angles around each carbon atom, making the bond angles 120o.
Ah thankyou! This is the explanation I was looking for
Original post by Big-Daddy
Hmm, so are the bond angles in any cyclic molecule still tetrahedral (if it's Carbon)? e.g. Butane - bond angle of 109.5 rather than 90? Pentane - bond angle of 109.5 rather than 108? C12H24 - bond angle of 109.5 rather than 150? I thought the shape of the cycle defined the bond angle ...
Alkane rings are not flat (apart from cyclopropane) - they are puckered to allow the bond angles to be closer to what the hybridisation state wants to be.
(edited 11 years ago)
Original post by thegodofgod
In cyclohexane, each carbon atom is bonded to four other atoms, (two carbon, two hydrogen) this makes it a tetrahedral structure if you think of the shape of the bonding around each carbon atom. Hence, the bond angles is 109.5o.
In benzene, each carbon atom is bonded to three others atoms, (two carbon, one hydrogen) this makes it a trigonal planar structure in terms of the bond angles around each carbon atom, making the bond angles 120o.
In benzene, each carbon atom is bonded to three others atoms, (two carbon, one hydrogen) this makes it a trigonal planar structure in terms of the bond angles around each carbon atom, making the bond angles 120o.
This works in terms of VSEPR but I thought the rotation in the ring was fixed, so it will act like a polygon in terms of bond angles? I'm talking about C-C-C bond angles here, not sure about which ones you're talking about ...
Original post by illusionz
Alkane rings are not flat (apart from cyclopentane) - they are puckered to allow the bond angles to be what the hybridisation state wants to be.
Are these C-C-C bond angles or H-C-C bond angles? I imagine the C-C-C bond angles should be 180(n-2)/n for cycloalkane CnH2n, whereas the H-C-C bond angles should be tetrahedral or trigonal planar or whatever as VSEPR dictates ...
Original post by Big-Daddy
Are these C-C-C bond angles or H-C-C bond angles? I imagine the C-C-C bond angles should be 180(n-2)/n for cycloalkane CnH2n, whereas the H-C-C bond angles should be tetrahedral or trigonal planar or whatever as VSEPR dictates ...
Read this http://en.wikipedia.org/wiki/Cyclohexane_conformation
Original post by illusionz
Hmm ok I see, but can they maintain the tetrahedral bond angle through those (or similar confirmations) as n gets larger? e.g. for C20H40, will it still be 109.5? Or does it "give up" at some point and just behave like a polygon?
Original post by Big-Daddy
Hmm ok I see, but can they maintain the tetrahedral bond angle through those (or similar confirmations) as n gets larger? e.g. for C20H40, will it still be 109.5? Or does it "give up" at some point and just behave like a polygon?
It is more favourable to keep the bond angles tetrahedral as this is where the orbitals are. Larger rings are more flexible and hence there is less strain in doing this than say, cyclobutane.
Original post by illusionz
It is more favourable to keep the bond angles tetrahedral as this is where the orbitals are. Larger rings are more flexible and hence there is less strain in doing this than say, cyclobutane.
I see, again. Thanks for illuminating me!
So what are the bond angles in cyclopropane, cyclobutane and cyclopentane, then? I'm guessing C-C-H is still 109.5 in each, but C-C-C is the question ...
Original post by Big-Daddy
I see, again. Thanks for illuminating me!
So what are the bond angles in cyclopropane, cyclobutane and cyclopentane, then? I'm guessing C-C-H is still 109.5 in each, but C-C-C is the question ...
So what are the bond angles in cyclopropane, cyclobutane and cyclopentane, then? I'm guessing C-C-H is still 109.5 in each, but C-C-C is the question ...
60ish (but there are theories suggesting the bonds are bent and not straight due to the shape of sp3 orbitals), 90ish, 109.5
Original post by Kev.1995
Why is it 120 in benzene but 109.5 in cyclohexane?
Which part of AQA Chem is this?
Which part of AQA Chem is this?
Good question kev
do we need to know this for our course?
Original post by illusionz
Alkane rings are not flat (apart from cyclopentane) - they are puckered to allow the bond angles to be closer to what the hybridisation state wants to be.
Cyclopentane is not flat (planar)
Original post by charco
Cyclopentane is not flat (planar)
Sorry I meant to say cyclopropane. The other one beginning with p!
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