A Level Organic Chemistry Questions!

Hi, i'm slightly unsure how to answer these few questions, no, infact i'm completely unsure how, please help.... :smile:

1. Boiling points of water, ethanol and ethoxyethane are in the reverse order if their relative molecular masses, unlike those of their analogous sulphur compounds, H2S, C2H5SH and C2H5SC2H5. WHY?

2. BF3 is non-polar, NF3 is polar. WHY?

Reply 1

belu_bustu

1

Hi,

1.
The key thing here is the difference in electronegativity between oxygen and sulphur. Oxygen is highly electronegative. The result is that the dipole moment of the O-H bond is very big: so much so that very strong permanent dipole-dipole attractions (called hydrogen bonding) can occur between -OH groups. A molecule of water has effectively two hydroxyl groups, and so can form twice as many hydrogen bonds as ethanol. Ethoxyethane cannot undergo hydrogen bonding. However the molecule is v-shaped, and still has a considerable net dipole moment because oxygen is still more electronegative than carbon. Hence molecules of ethoxyethane can interact via permanent dipole attractions.

Sulphur has a similar electronegativity to carbon and hydrogen. Hence H2S forms very weak permanent dipole-dipole attractions. These are weaker still in ethanethiol, but van der Waals forces compensate this due to the larger carbon skeleton. Finally the thioether has the smallest net dipole moment, but the largest carbon skeleton: thus the greatest intermolecular van der Waals attractions.

2.
Although the dipole moment of the B-F bond is greater than the N-F bond, the BF3 molecule is trigonal planar in shape and so has NO net dipole moment. NF3 however has a trigonal pyramidal structure, and so DOES have a net dipole moment. Hence NF3 molecules can are polar overall, whereas BF3 molecules are not.

Hope this helps.

Reply 2

What's Chico Time Precious?

OP

12

Yes that does help greatly!! :smile:

Perhaps i could trouble you further.... ?

3. The ionic nature of MgCl2 is greater than that of AlCl3, which in turn is greater than that of SiCl4. WHY?

4. Silicon (iv) oxide is a solid at room temperature that does not melt until 1973k, whereas CO2 melts at 217k and is a gas at room temp. WHY?

I'll rep you for the help already but i don't think i can give out anything major. Thanks anyway for the help! :smile:

Reply 3

dinesh

Generally even if bonds are polar in a molecule, if they are balanced and the molecule is symmetrical there is no overall dipole.

It is also wise to note that even if a molecule has permanenet van der waals forces it doesn't mean it's molecules are held together stronger than that of a dispersion force. Usually the dispersion forces predominate.

Reply 4

dinesh

Absolution

Yes that does help greatly!! :smile:

Perhaps i could trouble you further.... ?

3. The ionic nature of MgCl2 is greater than that of AlCl3, which in turn is greater than that of SiCl4. WHY?

4. Silicon (iv) oxide is a solid at room temperature that does not melt until 1973k, whereas CO2 melts at 217k and is a gas at room temp. WHY?

I'll rep you for the help already but i don't think i can give out anything major. Thanks anyway for the help! :smile:

That has to do with electronegativity. The small size of the Al3+ and high charge makes it a very good polariser ( ability to attact the charge cloud of electrons around a molecule/ion).
The colvanet character is this increased. The greater the diff in electronegativities between the molecules the more ionic it is.

Si02 has a macromolecular strucutre, each SI bonded tetrahedrally to 4 others. On melting these strong sigma bonds need to be broken, as the struccutre is destroyed.
IT has a diamond structure.

Co2 is a simple molecule with only permanent van der waals and VDW disperson forces, hence little energy is required to break apart it's molecules.

Reply 5

phishfood

can you see that Al3+ is smaller than Mg2+ and has a bigger charge? this means that it pulls the electrons on the Cl- ion toward it by a lot more, effectively pulling the electrons almost into a covalent bond as opposed to an ionic attraction (i.e. polarisation). Mg2+ is therefore more ionic and less covalent, and the same argument applies for SiCl4.

with SiO2 and CO2, think about the forces you have to break to melt them. SiO2 is a giant structure of covalently bonded atoms, whereas CO2 exists in small molecules with only van der Waals forces holding them together in a gas