Dispersion and polar bondings (1 Viewer)

[Steven12]

Hi, i just want to clear up on thing in ethanol and its bondings. so if anyone can correct me.


ethanol has dipole dipole bondings with other polar molecules because its OH group is electro negative.

ethanol has dispersion forces between its carbon to carbon chain(dues to unsymmetrical distripution of electron at any given instance). longer the chain, stronger the dispersion force

ethanol also has hydrogen bonding due to the surrounding hydrogens., which can attach itself to another polar group such as H (-OH-).

so why does ethanol evaporate before water?>
is it because water has stronger hydrogen bonding than ethanol.(whats the difference?)

or is it because ethanol has weaker bonding with surrounding molecules due to whatever reason>?

can someone help me?

 

[CM_Tutor]

Ethanol has dispersion forces because all molecular substances have dispersion forces - all that is needed for dispersion forces is molecules that have electrons (ie all molecules). As molar mass increases (and longer chains is one example of how this might occur), there are more electrons, and hence the number and overall strength of dispersion forces increases.

Ethanol has dipole-dipole interactions because it is polar - it is polar because there are polar bonds (the C-O and O-H bonds are polar as there is a sufficiently large difference in electronegativity between the ends of each of these bonds) and the geometry - bent about the O atom - is such that the individual bond dipoles reinforce, producing a net dipole on the molecule.

Ethanol has hydrogen bonding as it satsfies the criteria required by the definition. To have hydrogen bonding, there needs to be at least one H atom covalently bonded to a very electronegative atom (N, O or F) and another very electronegative atom with a lone pair. In other words, you need:

(N, O or F)-H ... : (N, O or F) where ... is the hydrogen bond and : is the lone pair

Ethanol has all the required criteria for possessing hydrogen bonding.

Ethanol has an average of 1 x H-bond per molecule, as it only has one H atom that can act as a hydrogen bond donor.

Water has an average of 2 x H-bonds per molecule, as each water molecule has both two H atoms that can act as hydrogen bond donors, and two lone pairs on the O atom that can act as hydrogen bond acceptors.

In comparing water and ethanol, the greater hydrogen bonding in water means that more energy will be required to vapourise water, and hence it will have a higher boiling point. (Aside - the stronger dispersion forces will actually occur in ethanol, due to its greater molar mass, but these will not be nearly strong enough to counter the effect of the greater hydrogen bonding of water.)

 

[Steven12]

cool.

so ethanol has a lower boiling point mainly due to its weak hydrogen bonding with surrounding ethanol molecule, though it may have stronger dispersion(which is very weak intermolecular force, which doesnt amount too much). But does the electronegative CO and OH amount to anything? wouldnt it create a dipole dipole pairing with other ethanol?

and i want to clear other things as well.

for the order of boiling point from highest to lowest

it is
Alkanoic acid, alkanol, and alkane Right?

alkanoic acid has a higher boiling point dues its high overall electronegativity.CO OH. is it?
alkanol is second highest since it has a polar end

alkane is the weakest since alkane uses dispersion force between molecule?

 

[Tommy_Lamp]

alkanoic acid has the highest melting and boiling point of the three because it has TWO sites for hydrogen bonding, alkanols have one site and alkanes have none

 

[Steven12]

how do you determine which site the molecule allows hydrogen bonding?

 

[Tommy_Lamp]

it forms between an OH and an O

tho there may be some exceptions, but youll have to ask CM_Tutor about that

 

[withoutaface]

I thought it was only one site, and the -OH group serves for dipole-dipole forces.

 

[xiao1985]

how do you determine which site the molecule allows hydrogen bonding?

usually, if you have H bonded to either a F, O or N, u have urself a hydrogen bonding...

keep in mind that hydrogen bonding is just an extremity of dipole dipole interaction... so a highly electronegative atom (say O) when bonded to a moderate electronegative atom (say C) can also have a dipole moment strong enough to contribute to the mp, bp difference...

 

[CM_Tutor]

It is alkanoic acid > alkanol > alkane.

Alkane has dispersion forces only.

Alkanol has dispersion forces, dipole-dipole interactions and H-bonds (average 1 per molecule). Due to greater molar mass, alkanol's dispersion forces are stronger than alkanes.

Alkanoic acid has dispersion forces, dipole-dipole interactions and H-bonds (average 2 per molecule). Alkanoic acid has greater dipole-dipole interactions than alkanol, as size of dipole is greater. Alkanoic acid also has greater dispersion forces than alkanol, due to greater molar mass.

Since strength of intermolecular interactions determines amount of heat required for vapourisation, and hence BP, and since alkanoic acid has the strongest interactions, and alkane the weakest, the order is

alkanoic acid > alkanol > alkane​

for same chain length.