organic question...

[faerielynx]

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what happens to the melting pt. and boiling pt. in alkene, alkynes and alkanes?😕

 

[uvaGirL07]

for alkanes...

increase in chain length = increases in BP, MP, and density
increase in branching = decreases in BP, MP, and density

i think it's supposed to be similar for alkenes and alkynes...

 

[rose786]

for alkanes...

increase in chain length = increases in BP, MP, and density
increase in branching = decreases in BP, MP, and density

i think it's supposed to be similar for alkenes and alkynes...

According to my EK book, Increased branching = decreased BP and increased MP. 😕

 

[uvaGirL07]

According to my EK book, Increased branching = decreased BP and increased MP. 😕

hmmm in kaplan they definitely say:

increased branching = decreased BP, MP, and density for alkanes.
(page 286 in the blue book that they give with the course)

and for the alkenes and alkynes they just say that they have similar properties (as the alkanes)... soooooooo who to trust.... kaplan or examkrackers... haha 😱

 

[uvaGirL07]

I think Kaplan was wrong... again... 🙄

Increased branching leads to decreased BP, but increased MP... silly kaplan. 👎

 

[faerielynx]

i think branching makes the melting point lower... where did you see it differently?...

 

[uvaGirL07]

I asked my brother who is taking orgo right now. He showed me a lecture slide from his class that said: "Branching tends to raise the melting point and lower the boiling point." I hope his professor is right. haha

 

[faerielynx]

okay... i suppose i'll go with that then... hope it's right!🙂

 

[joonkimdds]

According to my EK book, Increased branching = decreased BP and increased MP. 😕

if BP decrease, shoudln't MP decrease too?

 

[faerielynx]

...so which is it?

 

[raya23]

increased branching leads to decreased boiling point, increased melting point

 

[rose786]

increased branching leads to decreased boiling point, increased melting point

This is correct. I had to go back and double check my EK book. Sorry if I confused anyone.

 

[Hope2007]

Ohh, I love Organic Chemistry (I was scored 25)

Increase Branching in molecules would decrease both mp and bp ( and I am possitive).
This makes sense because increasing branching would cause molucules less densely packed together, therefore it requires less heat to break the lattice. While increase branching would decrease the force between the molecules (van de waals forces) would result in lower bp.

 

[gomawum]

Increasing MW, Increasing # of H-bond, and decreasing branching increase BP
(overall, anything that increases 'INTERMOLECULAR FORCE' increase BP)

Increasing MW, Increasing Branch (if symmetric), decreasing branch (if asymetric) increase BP
(overall, anything that increase the 'packing' increase MP)

I've had same question, and read kaplan, ap ochem, exam krackers, and came to a conclusion as above.

correct me if I'm wrong

 

[AspiringOne]

Sorry to revive this....but this is really ridiculous. While the logic that increased branching would lead to a decreased m.p. makes sense, I have found lecture presentations online that specifically show molecules with increased m.p. upon branching.

What's the verdict on this one?

 

[Hope2007]

Sorry to revive this....but this is really ridiculous. While the logic that increased branching would lead to a decreased m.p. makes sense, I have found lecture presentations online that specifically show molecules with increased m.p. upon branching.

What's the verdict on this one?

Some articles/lectures are misleading. We really need to understand the concept. Chemistry is very logical (fact). If we are not sure about something, we need to look up in several different text books. I was reading this article online and they said that alkyne is weeker then alkene who is weaker than alkane. Base on my understanding, this is not correct. Alkyne is stronger than alkene because it contains 1 sigma bond + 2 pi bonds while akene only has 1 sigma bond+1 pi bond. Because of the addtion pi bonds (more electron delocolized) makes it unstable and therefore very reactive. Thus The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol and the second pi bond is weak with 202 kJ/mol bond strength.
Increasing in order of MP and BP (assume they have similar MW and MF)
alkane<alkene<alkyne

 

[tastybeef]

Some articles/lectures are misleading. We really need to understand the concept. Chemistry is very logical (fact). If we are not sure about something, we need to look up in several different text books. I was reading this article online and they said that alkyne is weeker then alkene who is weaker than alkane. Base on my understanding, this is not correct. Alkyne is stronger than alkene because it contains 1 sigma bond + 2 pi bonds while akene only has 1 sigma bond+1 pi bond. Because of the addtion pi bonds (more electron delocolized) makes it unstable and therefore very reactive. Thus The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol and the second pi bond is weak with 202 kJ/mol bond strength.
Increasing in order of MP and BP (assume they have similar MW and MF)
alkane<alkene<alkyne

What you just described has nothing to do with MP and BP. MP and BP depends on intermolecular forces and what you described is intramolecular forces. When something melts or boils, the molecules move further away from each other, breaking the intermolecular forces. The molecules themselves DO NOT break apart.

As for the question, I've never thought about it. I agree with the idea that increased branching increases both MP and BP, however it doesn't answer the OP's question, which focuses on the number of bonds between 2 carbons, not branching. I would venture a guess to say that in hydrocarbons of similar MW and structure, BP and MP in increasing order is alkane, alkene, and alkyne. The pi orbital(s) in the double bond and triple bond allow molecules to become more tightly packed. The rationale behind my guess is kinetic stability that is also prevalent in many reactions such as Diels-Alder and Sn2 at the benzylic position. What you notice about these reactions is that the pi bonds from the starting material overlap to allow for a stable transition state (kinetically stable) and the reaction proceeds forward. This is especially well documented in Diels-Alder with endo directing molecules, which usually have extra double bonds allowing for extra stabilization with the diene.

 

[Hope2007]

What you just described has nothing to do with MP and BP. MP and BP depends on intermolecular forces and what you described is intramolecular forces. When something melts or boils, the molecules move further away from each other, breaking the intermolecular forces. The molecules themselves DO NOT break apart.

As for the question, I've never thought about it. I agree with the idea that increased branching increases both MP and BP, however it doesn't answer the OP's question, which focuses on the number of bonds between 2 carbons, not branching. I would venture a guess to say that in hydrocarbons of similar MW and structure, BP and MP in increasing order is alkane, alkene, and alkyne. The pi orbital(s) in the double bond and triple bond allow molecules to become more tightly packed. The rationale behind my guess is kinetic stability that is also prevalent in many reactions such as Diels-Alder and Sn2 at the benzylic position. What you notice about these reactions is that the pi bonds from the starting material overlap to allow for a stable transition state (kinetically stable) and the reaction proceeds forward. This is especially well documented in Diels-Alder with endo directing molecules, which usually have extra double bonds allowing for extra stabilization with the diene.

Yes. I agree with what you said. Sorry for misunderstood at some points in my previous writing. I mentioned the bond energy of the pi bonds and sigma bonds because I was refering to the article that I'd read online. They said that alkyne is weaker than alkene who is weaker than alkane. Yes. I definitely agree that the mp and the mp depends on the strength of the attractive forces that hold the molecules close to each other. According to one organic book, Van der Waals forces or London forces held molecules of alkane (alkene or alkyne) together. The greater the area of contact, the stronger the van der Waals forces and the greater the amount of energy needed to overcome these forces which exits between these molecules which you called it as "intermolecular forces" 🙂

 

[Corky Cacas]

what happens to the melting pt. and boiling pt. in alkene, alkynes and alkanes?😕

whent number of branching increases in alkane or alkene, boiling point and melting point decreases. but becareful when it is asked about alkene in terms of cis and trans. cis has a higher boiling point and lower melting point because of polarity. trans has a higher melting point and lower boiling point because of symmetry.

Now when there is more branching molecules are held together less tightly and this brings lower melting and boiling temp. also know that as number of branching increases it reduces the intermolecular attraction (van der waals forces).
i hope this will help you.🙂

 

[Corky Cacas]

what happens to the melting pt. and boiling pt. in alkene, alkynes and alkanes?😕

increased branching leads to decreased boiling point, increased melting point

wrong. increase in branching leads to decrease in MP and BP. melting and boiling point defere when it is talked about in the relm of cis and trans.

 

[joonkimdds]

Ohh, I love Organic Chemistry (I was scored 25)

Increase Branching in molecules would decrease both mp and bp ( and I am possitive).
This makes sense because increasing branching would cause molucules less densely packed together, therefore it requires less heat to break the lattice. While increase branching would decrease the force between the molecules (van de waals forces) would result in lower bp.

so this means that I was right and I deserve to score 25 or higher on OCHEM. Thanks for boosting my self-esteem 🙂