C-O, C-N, and ionic bond strength

[Gilakend]

I'm working through EK Chem book and it says since nitrogen will share electrons more with carbon it will make a stronger, less polar, less reactive bond than a C-O bond. I would have thought that the more polar, the shorter and stronger the bond.

If C-N is stronger than C-O because it shares it's electrons more, why are ionic bonds so strong when they are very polar?

Thanks!

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[AdaptPrep]

Think of the carbon-carbon single bond. The electrons will spend equal amount of time between each carbon which makes for a strong bond. If you consider the carbon-nitrogen bond, the electrons will spend a little more time on the nitrogen. Even though the bond is shorter (that is because N has a smaller radius than C), the bond is weaker. It would be weaker because electrons are not shared equally. If you consider the carbon-oxygen bond, the electrons will spend even more time closer to the oxygen, so that bond would be weaker still (again, the bond may be shorter due to smaller radius, but the unequal sharing of the electrons make it weaker). This is observed in organic chemistry...weaker bases make better leaving groups. Hence, -OH would be easier to remove from a carbon than -NH2 (of course, adding an acid makes it a better leaving group). This is because the C-N bond is stronger than a C-OH bond. This is the trend going across the same period. If you go down a group, the bonds get longer and weaker(HF<HCl<HBr<HI) due to increasing bond length.

Once you are talking ionic bonds (metal bonding with non-metal like sodium chloride), you no longer have covalent bonds but ionic attractions between fully charged anions and cations. The ionic attraction can be quite strong!

 

[Gilakend]

Think of the carbon-carbon single bond. The electrons will spend equal amount of time between each carbon which makes for a strong bond. If you consider the carbon-nitrogen bond, the electrons will spend a little more time on the nitrogen. Even though the bond is shorter (that is because N has a smaller radius than C), the bond is weaker. It would be weaker because electrons are not shared equally. If you consider the carbon-oxygen bond, the electrons will spend even more time closer to the oxygen, so that bond would be weaker still (again, the bond may be shorter due to smaller radius, but the unequal sharing of the electrons make it weaker). This is observed in organic chemistry...weaker bases make better leaving groups. Hence, -OH would be easier to remove from a carbon than -NH2 (of course, adding an acid makes it a better leaving group). This is because the C-N bond is stronger than a C-OH bond. This is the trend going across the same period. If you go down a group, the bonds get longer and weaker(HF<HCl<HBr<HI) due to increasing bond length.

Once you are talking ionic bonds (metal bonding with non-metal like sodium chloride), you no longer have covalent bonds but ionic attractions between fully charged anions and cations. The ionic attraction can be quite strong!

Thank you very much, it all makes sense now!

 

[AdaptPrep]

Thank you very much, it all makes sense now!

Great! You are welcome. Good luck in your studying!

 

[deleted647690]

Think of the carbon-carbon single bond. The electrons will spend equal amount of time between each carbon which makes for a strong bond. If you consider the carbon-nitrogen bond, the electrons will spend a little more time on the nitrogen. Even though the bond is shorter (that is because N has a smaller radius than C), the bond is weaker. It would be weaker because electrons are not shared equally. If you consider the carbon-oxygen bond, the electrons will spend even more time closer to the oxygen, so that bond would be weaker still (again, the bond may be shorter due to smaller radius, but the unequal sharing of the electrons make it weaker). This is observed in organic chemistry...weaker bases make better leaving groups. Hence, -OH would be easier to remove from a carbon than -NH2 (of course, adding an acid makes it a better leaving group). This is because the C-N bond is stronger than a C-OH bond. This is the trend going across the same period. If you go down a group, the bonds get longer and weaker(HF<HCl<HBr<HI) due to increasing bond length.

Once you are talking ionic bonds (metal bonding with non-metal like sodium chloride), you no longer have covalent bonds but ionic attractions between fully charged anions and cations. The ionic attraction can be quite strong!

If this is true, then why is the IR absorbance value for an alkene around 1620 cm^-1 while the IR absorbance for a carbonyl C=O bond is around 1700 cm^-1? This would imply that a CC bond is in fact WEAKER than a C=O bond, since less energy is required to stretch the C=C bond than the C=O bond.

 

[Cornfed101]

C=C is weaker than C=O.

 

[deleted647690]

View attachment 284213

C=C is weaker than C=O.

How does the reasoning above from @AdaptPrep for single bonds apply? According to what he said, a bond between two different atoms will become weaker as the electronegativity difference increases, because the bond electrons are more unequally shared. I would think this would suggest a C=O bond would be weaker than a C=C bond

 

[Dochopeful13]

How does the reasoning above from @AdaptPrep for single bonds apply? According to what he said, a bond between two different atoms will become weaker as the electronegativity difference increases, because the bond electrons are more unequally shared. I would think this would suggest a C=O bond would be weaker than a C=C bond

Following

 

[Cornfed101]

How does the reasoning above from @AdaptPrep for single bonds apply? According to what he said, a bond between two different atoms will become weaker as the electronegativity difference increases, because the bond electrons are more unequally shared. I would think this would suggest a C=O bond would be weaker than a C=C bond

It’s been a while, and I know I learned everything that affects bond strength at some point, but I am likely missing one or two things. I recommend looking in your ochem textbook or asking your professor for more clarification.

First, the bond energy table I provided above always tells which bonds are stronger. Therefore C=O is stronger than C=C. Trust data before anything else.

As electronegativity difference increases the bond strength increases. This is because it is harder to pull the more electronegative off. I think you are confusing two separate trends (C-O compared to C-C, vs C-F compared to C-Cl)

As you go down a column in the periodic table the bond becomes weaker because you are adding more shells and more distance. This distance is also why double bonds are stronger than single bonds. Double bonds are shorter.

The stability of the conjugate base and resonance also play roles. Basically the more stable something is the stronger the bond, but that is more of a macro idea than the original question.

 

[PlsLetMeIn21]

How does the reasoning above from @AdaptPrep for single bonds apply?

It doesn't. C-C is 347 kJ/mol, C-N is 305 kJ/mol, and C-O is 358 kJ/mol. C=C is 614 kJ/mol, C=N is 615 kJ/mol, and C=O is 745 kJ/mol. The trend is not consistent, and sigma bonds between C and N are an exception. I recall my professor talking about an orbital overlap issue between C and N in the sigma bond but that the pi bonds between C and C, N, and O followed the trend. My TBR book says that more electronegative atoms tend to hold on to shared electrons more tightly and make breaking that bond more difficult. That seems consistent with all of the numbers I just read in the energy table except for C-N sigma bonds.

 

[PlsLetMeIn21]

Think of the carbon-carbon single bond. The electrons will spend equal amount of time between each carbon which makes for a strong bond. If you consider the carbon-nitrogen bond, the electrons will spend a little more time on the nitrogen. Even though the bond is shorter (that is because N has a smaller radius than C), the bond is weaker. It would be weaker because electrons are not shared equally. If you consider the carbon-oxygen bond, the electrons will spend even more time closer to the oxygen, so that bond would be weaker still (again, the bond may be shorter due to smaller radius, but the unequal sharing of the electrons make it weaker).

You might need to rethink your reasoning here. I believe you have it backwards for homolytic bond breaking. C-O has the highest bond dissociation energy of the bonds you mentioned, because it is not as willing to give the shared electron back to carbon when the bond breaks.