Question

can someone explain this to me

Answer 1

So, a double bond has a stronger bond then single and triple stronger then double and single. So how come its easier to break them apart if its stronger? like for example, alkynes are the most reactive out of alkanes and alkenes

Answer 2

This is a good question! The simple answer is this: if you consider the TOTAL bond energy of a double bond or triple bond, it's going to be higher than that of a single bond. The INDIVIDUAL bond energy of the second (in a double or triple bond) and third (in a triple bond) bond is less than that of a the first bond (or just a single bond). This makes these additional bonds easier to break, making them more reactive.

The more complicated answer comes down to the geometric configuration of the bonds themselves, and specifically the orbitals that form the bonds. In the simplest case, you can think of three mutually perpendicular oribtals. Single bonds are formed by "sigma" bonds, bonds in which the orbitals of the bonding atoms overlap directly. The large area of overlap makes these sigma bonds relatively strong. In a double or triple bond, one of the two or three bonds is a strong sigma bond. The additional bonds are so-called "pi" bonds. In these bonds, the bonding orbitals overlap only a little, meaning they are relatively weaker and easier to break. Have a look at the picture at the top of this page to get an idea of what I mean: http://butane.chem.uiuc.edu/pshapley/GenChem2/A6/3.html.

And if you have any questions please ask!

Answer 3

1.so for double and triple its total bond energy is greater then that of single but its distributed (if you can put it that way) to the second and third(if triple) making it significantly easier to break because less energy within each individual bond (and it can be called individual bond right? )

therefore the order form highest to lowest reactivity is, alkyne, alkene, alkane?

2.so this knowledge could be applied to boiling points right? the harder it is to break them apart the higher the boiling point because of stronger intermolecular forces right?

therefore the order of highest to lowest BP is alkane, alkene and alkyne?

3. what about the cyclo-____ so my notes say that because of its STRAINED structure it has a higher boiling point then alkane..... what does a strained structure mean.

and i'm guessing that cyclo alkyne and alkene would be more reactive the cycloalkane because of their multiple bonds? and cycloalkane would have the highest BP out of the other two?

4. My notes also said that there is not alot of known cycloalkenes and cycloalkynes because of the bond angle stress put on the double or triple bonds?

what is B.A.S?


I KNOW ITS ALOT OF QUESTIONS TO ANSWER but i really need to ace chem, and i wont ace it if i dont understand (saying sorry in advance and future questions)
@matt101 - you answer was very good by the way

Answer 4

Yes that's pretty much it. The total bond energy for a multiple bond is greater than for a single bond. But keep in mind that the additional bonds beyond a single bond are all equivalent (they are all pi bonds of equal strength). These additional bonds are the reactive bonds because they are weaker. And yes you can refer to them as individual bonds.

Both alkynes and alkenes are more reactive than alkanes. If I remember correctly, however, alkenes are MORE reactive than alkynes as well. It has to do with the stability of the intermediates formed during the reaction (probably beyond the scope of what you need to know).

Yes, this can help you understand boiling points...but not for the reason you're thinking! When you boil something, you're changing the state, not the compound. This means you wouldn't actually be breaking any of the bonds (even if it's a double or triple bond). We need to think about the INTERmolecular bonds (between molecules). Double and triple bonds are INTRAmolecular bonds (within molecules). The specific order of which has the highest boiling point will vary based on the molecules we're talking about, because the polarizability of the multiple bonds as well as the geometry of the actual molecule will affect how strong the intermolecular forces are and determine the order of boiling points. It's not always going to be alkynes < alkenes < alkanes.

When we talk about ring strain we're talking about the bond angles of the atoms that make up the ring. When atoms are bonded together, they naturally form certain bond angles so that all atoms are as far apart as possible (and so most stable). However, when atoms are part of a ring, their ability to produce these angles is restricted, giving rise to strain. The further the actual bond angles are from the "ideal" angles (if the atoms weren't in a ring), the more strain there is. Cyclohexane has the least ring strain, because the atoms in it are able to form bond angles very close to the ideal. As you add or subtract carbon atoms to the ring, the bond angle moves away from this ideal and ring strain increases.

Yes, cycloalkenes and cycloalkynes would be more reactive. But the specific order of boiling points might be a bit more challenging for the same reason I mentioned before. If your teacher wants you to know a specific pattern I suggest you ask them.

Bond angle stress is what I described above. Double and triple bonds can sometimes make the stress worse, since these multiple bonds pull the atoms closer together. This adds another factor that affects the bond angle and resulting stress on top of just being part of the ring.

Whew...hope that's sort of helpful at least! Let me know if anything's unclear.

Answer 5

ok so for boiling point basically when boiling, your breaking the molecules not the bonds

and that the vsepr shape and polarity of the molecule as well as electrons and molecular weight/molarmass will affect the intermolecular force (being dipole dipole and london dispersion ) hence affecting boiling point right?

Answer 6

@matt101

Answer 7

Right. The molecules stay intact, they just separate from one another. And yes.

Answer 8

i was wondering if you could give me some examples of different alkanes alkenes and alkynes, and tell me to find which one is highest or lowest and what not? @matt101

Answer 9

atcually nevermind!