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John Doe III
science forum addict
Joined: 09 May 2005
Posts: 52
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 Posted: Wed May 18, 2005 5:57 pm Post subject:
melting point vs boiling point
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For melting point is the most important factor the shape of a given compound
(e.g. how compounds stack up or how compact they can be)? For example, it
seems to me the melting point of a trans or E seems to be higher than the
melting point of a cis or Z even when cis seems to be slightly heavier or
more polar or even in some cases where a cis compound can form hydrogen
bonds and a trans compound cannot.
And for boiling point is the most important factor the polarity of a given
compound?
I don't have any reference for the above question but after solving a bunch
of chem. problems that's my understanding. I am NOT sure if the above rule
can be used as a general method to draw sound conclusions.
Thank you so much in advance. |
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Marvin
science forum Guru Wannabe
Joined: 24 Mar 2005
Posts: 224
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| Posted: Wed May 18, 2005 6:26 pm Post subject:
Re: melting point vs boiling point
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John Doe III wrote:
| Quote: | For melting point is the most important factor the shape of a given compound
(e.g. how compounds stack up or how compact they can be)? For example, it
seems to me the melting point of a trans or E seems to be higher than the
melting point of a cis or Z even when cis seems to be slightly heavier or
more polar or even in some cases where a cis compound can form hydrogen
bonds and a trans compound cannot.
And for boiling point is the most important factor the polarity of a given
compound?
I don't have any reference for the above question but after solving a bunch
of chem. problems that's my understanding. I am NOT sure if the above rule
can be used as a general method to draw sound conclusions.
Thank you so much in advance.
Sorry, your "understanding' is a misunderstanding. If that is what your textbook says, get |
a different book.
Both boiling and melting points are affected by forces between atoms or molecules. Shape
does enter in, but is not THE determinent. And polarity can provide a binding force, but
only one of several kinds of force. |
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Ron Jones
science forum Guru Wannabe
Joined: 16 May 2005
Posts: 175
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| Posted: Wed May 18, 2005 6:31 pm Post subject:
Re: melting point vs boiling point
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John Doe III wrote:
| Quote: | For melting point is the most important factor the shape of a given
compound (e.g. how compounds stack up or how compact they can be)?
For example, it seems to me the melting point of a trans or E seems
to be higher than the melting point of a cis or Z even when cis seems
to be slightly heavier or more polar or even in some cases where a
cis compound can form hydrogen bonds and a trans compound cannot.
And for boiling point is the most important factor the polarity of a
given compound?
I don't have any reference for the above question but after solving a
bunch of chem. problems that's my understanding. I am NOT sure if the
above rule can be used as a general method to draw sound conclusions.
Thank you so much in advance.
|
Never rely on a melting point IMHO. As an expert on crystalliztion once
told me - at least 60-70% of organic compounds have polymorphs, and if you
can't find a second polymorph of a compound, then it just means you haven't
tried enough solvents! I have seen people in blind panic because the
melting point suddenly plunged 10C on a new batch of pharma intermediate -
and we *never* managed to make the higher melting polymorph ever again...
--
--
Ron Jones
Don't repeat history, see unreported near misses in chemical lab/plant
at http://www.crhf.org.uk |
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John Doe III
science forum addict
Joined: 09 May 2005
Posts: 52
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| Posted: Wed May 18, 2005 7:11 pm Post subject:
Re: melting point vs boiling point
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| Quote: | Sorry, your "understanding' is a misunderstanding. If that is what your
textbook says, get
a different book.
Ok, thanks, I guess the cases, that I have looked at, are exceptional cases |
and therefore no general pattern can be deduced. |
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Bob
science forum addict
Joined: 30 Apr 2005
Posts: 80
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| Posted: Wed May 18, 2005 11:00 pm Post subject:
Re: melting point vs boiling point
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On Wed, 18 May 2005 19:57:31 GMT, "John Doe III"
<john_doeIII@yahoo.com> wrote:
| Quote: | For melting point is the most important factor the shape of a given compound
(e.g. how compounds stack up or how compact they can be)?
|
Not really.
But MP are complex compared to BP. Boiling involves a change from a
phase with intermolecular forces to one without. MP involves a change
between two phases both of which have intermolecular forces. And shape
is much more of an issue in the structure of a solid than in the
structure of a liquid -- so in a sense your point has some merit.
I can’t imagine there is any generality about cis vs trans,
independent of the nature of the groups. That would be odd.
Go look up the MP of the alkane carboxylic acids of 1-6 carbons. It is
a good example of a shape effect for MP. (For fairness, also look at
the BP.)
bob
| Quote: | For example, it
seems to me the melting point of a trans or E seems to be higher than the
melting point of a cis or Z even when cis seems to be slightly heavier or
more polar or even in some cases where a cis compound can form hydrogen
bonds and a trans compound cannot.
And for boiling point is the most important factor the polarity of a given
compound?
I don't have any reference for the above question but after solving a bunch
of chem. problems that's my understanding. I am NOT sure if the above rule
can be used as a general method to draw sound conclusions.
Thank you so much in advance.
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Uncle Al
science forum Guru
Joined: 24 Mar 2005
Posts: 1226
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| Posted: Wed May 18, 2005 11:51 pm Post subject:
Re: melting point vs boiling point
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Bob wrote:
| Quote: |
On Wed, 18 May 2005 19:57:31 GMT, "John Doe III"
john_doeIII@yahoo.com> wrote:
For melting point is the most important factor the shape of a given compound
(e.g. how compounds stack up or how compact they can be)?
Not really.
But MP are complex compared to BP. Boiling involves a change from a
phase with intermolecular forces to one without. MP involves a change
between two phases both of which have intermolecular forces. And shape
is much more of an issue in the structure of a solid than in the
structure of a liquid -- so in a sense your point has some merit.
I can’t imagine there is any generality about cis vs trans,
independent of the nature of the groups. That would be odd.
Go look up the MP of the alkane carboxylic acids of 1-6 carbons. It is
a good example of a shape effect for MP. (For fairness, also look at
the BP.)
bob
For example, it
seems to me the melting point of a trans or E seems to be higher than the
melting point of a cis or Z even when cis seems to be slightly heavier or
more polar or even in some cases where a cis compound can form hydrogen
bonds and a trans compound cannot.
And for boiling point is the most important factor the polarity of a given
compound?
I don't have any reference for the above question but after solving a bunch
of chem. problems that's my understanding. I am NOT sure if the above rule
can be used as a general method to draw sound conclusions.
Thank you so much in advance.
|
Very high symmetries in a homologous seris of compounds will give
anomalously high melting points - neopentane, cubane, adamantane,
twistane, dodecahedrane, buckminsterfullerene - even though the
boiling points may not be exceptional. One then sees a lot of
sublimation.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf |
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muha
science forum Guru Wannabe
Joined: 08 May 2005
Posts: 167
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| Posted: Thu May 19, 2005 1:17 am Post subject:
Re: melting point vs boiling point
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"By means of symmetry we bring advancement in melting point to masses
in the countryside" Chairman Mao |
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