akimhaneul

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what determines the strength of covalent bonds that lead to bond energy values?

In the equation delta H = BE broken -BE formed, is the bond being broken heterolytic or homolytic?
 

JB50

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what determines the strength of covalent bonds that lead to bond energy values?

In the equation delta H = BE broken -BE formed, is the bond being broken heterolytic or homolytic?

Edited because I didn't contribute anything to OP's inquiry lol
 

aldol16

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what determines the strength of covalent bonds that lead to bond energy values?
The strength of covalent bonds depends on the degree to which the electrons are shared between the atoms involved in the bond.

However, when you're given a BDE, it's homolytic bond cleavage because that's simply how we measure it. So you run into interesting scenarios because of this discrepancy. For instance, take water and react it with a strong base. The base will deprotonate the water to form hydroxide and protonated base. Now take the same strong base and react it with isobutane. No reaction occurs. Now, O-H BDE is upwards of 100 kcals whereas C-H bond strength in this case is around 90 kcals. So based on that alone, you would expect the isobutane to be more reactive.

So what gives? Well, the key is that BDE is homolytic bond cleavage. So what it really measures is whether an isobutyl radical or hydroxyl radical is more stable! Because the isobutyl radical is more stable than the hydroxyl radical, the former will have a lower BDE. However, when you're reacting these with a strong base, you're not really forming a radical. You're doing heterolytic bond cleavage and that makes all the difference.
 
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akimhaneul

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The strength of covalent bonds depends on the degree to which the electrons are shared between the atoms involved in the bond.

Thank you! Do you mind explaining this part more? What do you mean by this?
 

aldol16

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Thank you! Do you mind explaining this part more? What do you mean by this?
So take an SN2 reaction between methyl iodide and generic nucleophile Nuc-. Nuc- attacks and iodide leaves. Why does iodide leave and not a hydride? Because the heterolytic bond cleavage energy for a C-I bond is less than for a C-H bond. I is much more electronegative than C so the electrons in that bond already reside closer to the I. In contrast, C and H have similar electronegativities so the electrons in that bond reside in the middle of the C-H axis. Since the C-I electrons are already closer to the I, it's easier to move those electrons completely towards the I, thereby breaking the bond.

So in terms of MO pictures, the C-I bond resembles the C-Cl bond below. The gap between the bonding MO and the Cl atomic orbital is small. The C-H bonds resemble the C-C bond below. The gap between the bonding MO and C atomic orbital is relatively larger. So the C-C bond is said to be stronger than the C-Cl bond in terms of heterolytic bond cleavage.