polar and non-polar (1 Viewer)

[wahashtini]

can sum1 tell me if im right:
alkene and alkanes are non-polar
alkanols (OH) are both polar and non-polar
and alkanoic acids are both

and one more favour, i need a bit more info on this dot point

• explain the difference in melting point and boiling point caused by straight-chained alkanoic acid and straight-chained primary alkanol structures

thanxx

 

[Paroissien]

Alkanoic more polar due to the presence of the =O bond, resulting in stronger dipole-dipole forces which means more energy is required to break the bonds. Therefore mp and bp is higher

 

[wahashtini]

can u clear this out for me, briefly wat r dipole-dipole forces??

 

[Paroissien]

Basically the attraction behind two molecules due to their opposing polarities. Like in water, the water molecules are bound together due to their opposing polarities, which results in a relatively high bp

 

[~*HSC 4 life*~]

dipole- think of DI (two) ends, one positive. one negative

it is related to an electronegativity of an element (recall: electronegativity is 'how badly it wants electrons') say, chlorine, a fairy highly electronegative element, can draw electrons to itself easily (only one e required to complete its valence shell) therefore, when it attracts an electron off say, hydrogen...it becomes slightly negative (recall electrons have a neg charge) and the hydrogen has a slightly positive end (as an electron is being shared/drawn to chlorine)

remember dipole dipole bonding is the 'unequal sharing of electrons'

also on ur pt about alkanoic acids, they have a high bpt/mpt because they have the ability to form TWO hydroegn bonds per molecule...ifu look at a piccie, it's got the double bond to Oxygen, and a hydozide group, which can form 2 hydren bonds when orientated next to another alkanoic acid

 

[persephone]

The boiling point of alkanes, alkanols and alkanoic acids all increase with increasing molecular mass because the increasing mass is due to increasing carbon chain length. The longer the carbon chain the more dispersion forces exist between molecules and the more energy is needed to separate them
As you go from alkanes to alkanoic acids the functional group becomes more polar. Only weak dispersion forces need to be overcome to boil alkanes. However alkanols have these as well as dipole-dipole forces and hydrogen bonds due to the hydroxyl functional group. Alknoic acids have similar dispersion forces and similar hydrogen bonds due to the hydroxyl group but they are more polar molecules owing to the double-bonded oxygen as part of the carbonyl functional group. This gives more polarity to the molecule and results in stronger dipole/dipole forces. These require the most energy to overcome.

this is a sample answer from a textbook.....

 

[aim54x]

hydrogen bonding in alkanols make them polar but they are also non polar due to the other end of the chain. enes and anes r non polar.

 

[Timmy-V]

so a di-alkanol and a di-alkonoic acid would just be completely polar?

 

[CM_Tutor]

Try and avoid saying that an alkanol (1-butanol, say) is both polar and non-polar. A substance cannot be both polar and non-polar. It can, however, have both polar and non-polar regions. In the case of 1-butanol, a polar substance, the colecule has both polar and non-polar regions - the hydroxyl group is polar and the carbon chain is non-polar.

In the case of an acid like oxalic acid (ethanedioic acid - HOOC-COOH), there is no non-polar region within this polar molecule.

 

[Paroissien]

No because you would still have the carbon chain