Before we get into the details that define bong length and bond strength in organic chemistry, let’s briefly summarize these two properties from the start as they remain important for all the types of bonds we will discuss.
So, remember this: the shorter the button, the stronger it is.
To understand the principles of bond strength and bond length associated with organic molecules, let’s first discuss the known data for hydrogen halides:
Bond strength increases from HI to HF, so HF is the strongest bond while HI is the weakest.
First, if we look at the periodic table, we can discover a pattern related to bond strength and atomic size.
Note that atomic size increases down the periodic table with fluorine, for example, it uses a hybrid sp3 orbital made up of its second shell orbitals to form bonds with hydrogen:
The other halogens are in the 3rd, 4th, and 5th rows of the periodic table and therefore, use larger orbitals during hybridization and therefore bond formation.
If we put them next to each other, we can use this display of bond length differences to describe bond strengths as well:
What we see is that as the atoms get bigger, the bonds become longer and weaker as well.
Longer bonds are the result of larger orbitals that assume less electron density and a lower percentage overlap with hydrogen orbitals.
This is what happens as we move down the periodic table and therefore, the H-X bonds become weaker as they get longer.
So, considering this, let’s now see how the length and strength of the C-C and C-H bonds are related to the state of the carbon atom.
Bond Strength and Electricity
The relationship between bond length and bond strength is very precise and works in many of the types of bonds you will see in organic chemistry.
The best known is probably the effect of electricity on bond strength.
For example, bonds decrease in the following series, although we would expect the C-N bond to be stronger than the C-O bond because nitrogen has a smaller atomic size than oxygen: CH3–F > CH3–OH > CH3–NH2.
Remember, as the difference between the electron forces of the bonded atoms increases, we change from non-polar to polar covalent bonds, where the electron density is not shared equally between the atoms and they are characterized by positive and negative charges.
Now, when the atoms have these partial charges, the bonding between them begins to achieve ionic character as well.
Ionic bonds are generally stronger than covalent bonds, which we can also see by their higher melting points.
Therefore, this should explain why the C-O bond (143 pm, 360 kJ/mol) is usually stronger and shorter than the C-N bond (147 pm, 305 kJ/mol) because the atomic sizes are not so different while the potential electronic giant. oxygen brings some ionic character to the C-O bond.
This is also true when comparing the bond strengths of O-H (97 pm, 464 kJ/mol) and N-H (100 pm, 389 kJ/mol).
Bond Length and Strength in Organic Molecules
Why do you think the C-H bond strength of alkane, alkene, and alkyne follows the pattern shown below?
We have concluded, in the previous section, that bond strength is inversely related to bond length, and based on this, the data suggest that the C-C bond in alkanes must be the longest as it is the weakest, and the C-C bond in alkynes is the shortest as it appears be more severe.
And this, in fact, is true because remember, the bond length decreases from sp3 to sp hybrid:
To understand this trend in bond lengths based on diversification, let’s quickly recall how diversification occurs.
The next question is – how the behavior of s is related to bond length and strength.
A smaller orbit, in turn, means a stronger interaction between the electron and the nucleus, a shorter and, therefore, stronger bond.
This is why the C-C bond in alkynes is the shortest/strongest, and that of alkanes is the longest/weakest as we saw in the table above.
C-C vs. C-H Bond Strength
The relative size of the s orbital also explains why the C-C σ bond is weaker than the C-H σ bond.
And that’s because hydrogen uses a “pure” orbital (the letter 100% s) which is closer to the nucleus than the sp3 orbital of carbon.
Now there are different types of C-H bonds depending on the combination of the carbon to which the hydrogen is attached.
As in all the examples we have discussed so far, the strength of the C-H bond here depends on the length and therefore on the carbon bond to which the hydrogen is attached.
The longer the s in the hybrid orbital connecting the two atoms, the shorter and stronger the C-H bond:
To summarize the information in the table, note the bond strength order C(sp)-H > C(sp2)-H > C(sp3)-H.
The opposite will be true regarding bond length.
All these values mentioned in the tables are called bond dissociation energy – that is the energy required to break a given bond.
In particular, we are talking about homolytic fission when each atom gains one electron when breaking a bond.
Bond dissociation forces common in organic chemistry as well as the homolytic dissociation mechanism (radical effect) will be covered in the next article which you can find here.
The power of Sigma and Pi bonds
There is one important point that we have to address when comparing the strength of a single bond with a double or triple bond.
Now, if we compare the strength of a single bond to a double bond, we have 88 kcal/mol :152 kcal/mol.
determine the combination of the respective atoms and indicate the shortest and strongest bond:
A short and strong bond is shown in red.
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