24

౪

1 Introduction

Figure 1.25: Realistic stress-strain curves.

propagations and statistical probabilities of failure are usually built into the mathemat-

ical models used.

1.3 Nanoscale Reactions

What are all the chemical reactions these nanoscale actors can perform? Basically, we

are talking about all of organic chemistry, inorganic chemistry, and biochemistry com-

bined…. Many of these individual reactions can be found in textbooks written for those

fields. Here we will only select a few important facts, principles, and reactions that will

help you understand the genius of some of the nanotechnology that will follow.

Let us start with some organic chemistry that will help with biochemistry as well, by

looking at the structure-function relationships of organic molecules. Some of the func-

tional groups on an organic or biomolecule are more reactive than others. There are

basically two types of structures that are reactive: Double and triple bonds, and polar

groups. The reactivity of multiple bonds comes from the type of bond involved: π-bonds.

π-bonds are formed by the overlap of p-orbitals. This overlap is energetically a lot less

than the overlap between s-orbitals that form σ-bonds. One should watch out for the fact

that a double bond consists of a σ-bond and a π-bond, not two π-bonds. Also, σ-bonds can

be formed by the overlap of hybridized orbitals, such as the sp³-hybridized orbitals that

from the bonds in CH4. π-bonds, though, can only be formed by the overlap of p-orbitals.

Thus they will always be weaker and more reactive.

The reactive functional groups have in common that they contain polar bonds. Po-

lar bonds are bonds between atoms with significantly different electronegativity or elec-

tron density, such as O–H and C=O. The atom with the higher electron density is always

the nucleophile, the one with the lower electron density the electrophile. Nucleophiles

also have electron pairs that can be used to form bonds, and the electrophile is the re-

ceiver of that bond. Hydrogen atoms are generally not considered electrophiles; it is

more important to know about hydrogens if a functional group is acidic or basic, i. e.,

if the proton can leave or get taken up. Polarity and acidity determine partial and full

charges; generally, the more charge there is, the more reactive the functional group.