By: Ben Thompson and Kevin Easterbrook
Sucrose aka Table sugar (C12H22O11)
The Covalent/Molecular Bond formed between the non metal atoms-oxygen, hydrogen and carbon atoms that make up a sucrose molecule, are all constructed due to the sharing of valence electrons of the atoms. Sucrose is formed through 45 single covalent bonds between various hydrogen, oxygen, and carbon atoms. Carbon is located under the 14th group on the periodic table, meaning it contains four valence electrons and requires four more electrons to satisfy its octet. Oxygen is located under the 16th group on the periodic table, meaning it contains six valence electrons and requires two more electrons to satisfy its octet. Hydrogen is under the 1st group, only containing one valence electron, and only requiring 1 more electron to complete its valence shell. In order for the atoms to obtain their full valence shell, the electrons of the atoms must share electrons. Non metals share their electrons as there is no electrostatic attraction between two equally charged atoms (i.e. like charges repel and unlike charges attract).
The carbon atoms are a part of the base of this molecule. They comprise some central atoms within the molecule, as these carbon atoms have an octet which allows them to bond with four other atoms; in addition to this, carbon is also a central atom because of its electronegativity (2.6). As a result of this, it will easily attract neighbouring electrons.It easily attracts electrons becuase it has only 2 shells (orbital) and 6 protons; this amount of protons and low amount of shielding causes attraction of other moleculeseasily0 because there is less distance and stronger attraction. Similar to this, Oxygen is also a central atom within sucrose because it too has an octet which allows it to be bonded to four other atoms and, has a high electronegativity difference of 3.4. Oxygen is also located in group 2 which means it also has 2 shells but more amount of protons. This results in Oxygen easily being able to attract electrons from nearby atoms.This also means that it will attract electrons from neighbouring atoms easily. The reason Hydrogen is not a central atom is because its valence can only contain a maximum of two electrons. Consequently, hydrogen can only bond with one other element making it a peripheral atom. Hydrogen is also a peripheral atom because it has the lowest electronegativity (2.2) out of Carbon and Oxygen.
Valence shell electron pair repulsion (VSEPR) shape is used to predict geometric shape of central atoms from the number of electron pairs surrounding the atoms. As previously stated, both oxygen and carbon are central atoms in a sucrose molecule. The stereometric shape of the carbon atoms in sucrose are a tetrahedral shape. This means that the atom has four bonding pairs to which all of the electrons are bonded. The stereometric shape of the oxygen atoms in sucrose are a bent tetrahedral shape. This means that the atom has two bonding pairs and two lone pair electrons. The bonding angles of the oxygen atom are that of 180º and the bonding angles of carbon atom are that of 90º.
The peripheral hydrogens in the diagram to the right shows this attraction to the water molecules and more specifically, the lone pairs of oxygen.
A sucrose compound is comprised of 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms (C12H22O11), to which a total of 136 valence electrons are distributed amongst the 45 atoms. Despite being a covalent compound, which typically have relatively low melting points; sucrose is a complex molecule bonded quite strongly which results in the melting point of sucrose being 186 ºC which is relatively high. This is a result of the 45 bonds created between each atom. There is so many bonds between each atom that it requires mass amounts of energy to separate these bonds into the individual elements, Oxygen, Hydrogen, Carbon. As stated previously, Sucrose is comprised of 136 valence electrons. Since there is such a high amount of electrons within this complex molecule, it causes london force to increase. London forces (intermolecular) are momentary dipoles which are created by a molecule through the movement of atoms within that molecule. Peripheral atoms within a molecule temporarily shift causing one end of the molecule to have a negative dipole and the opposite end to have a positive dipole. Electrons and london forces are directly proportionate which can be related to sucrose;136 valence electrons is going to create stronger momentary dipole-dipoles; the creation of these strong momentary dipoles is another reason why sucrose has a high melting point. In addition to this, london forces are a type of Van der Waal force; Van der Waal forces are intermolecular forces which are the reason molecules are able to remain in their states. There are three types of Van der Waal forces which are London Forces, Hydrogen bonding, and dipole-dipoles; if there were no Van Der Waal forces, substances would not be able to maintain their current state. Consequently, because sucrose is experiencing so many forces simultaneously, it is very reactive. Polarity, hydrogen bonding, dipoles, and london forces all contribute to this highly reactive molecule which will be explained below.
Although there are london forces present within sucrose, as london forces are everywhere, there are other forces present too. Sucrose also experiences permanent dipoles which are a result of polarity. Polarity is the unequal sharing of electrons within a molecule; in other words, the electrons tend to lean towards the more electronegative atom. In order for a molecule to have polarity it must have an electronegativity difference of 0.5-1.7. There is polarity within sucrose because of oxygen-hydrogen and oxygen-carbon. Oxygen-hydrogen is polar because a electronegativity of 1.2 is created (O=3.4 H=2.2-----> 3.4-2.2=1.2); this is a polar molecule. Oxygen-Carbon is also polar because it too creates a electronegativity difference between 0.5 and 1.7( O=3.4 C=2.6----->3.4-2.6=0.8). In addition to this, dipoles are also created by polarity. The more electronegative side of the atom (Polarity arrow points this way) forms a negative dipole because it is more electronegative. The other side of the molecule forms a positive dipole because it is less electronegative. Sucrose is soluble in water or, specifically polar solvents, because of this dipoles. The positive dipole attracts the negative dipole within the solvent while the negative dipole attracts the positive dipole within the solvent. This allows sucrose to be dissolved within the polar solvent.
Hydrogen bonding is also a pivotal part of sucroses solubility. Hydrogen bonding is the strongest intermolecular force that deals with hydrogen atoms and momentary attraction to lone pairs of other atoms. In order for hydrogen bonding to occur, two things must be present:
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Hydrogen must be bonded to a HIGHLY electronegative atom
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There must be at least one lone pair on the central atom the hydrogen is attracted to
The hydrogen atoms within sucrose (peripheral atoms) are positive dipoles as a result of the overall polarity of sucrose; this gives the hydrogen atoms positivity. However, hydrogen loses its attraction to this atom and becomes attracted to the lone pairs of another central atom. The central atom that hydrogen is now attracted to, forms a positive dipole which is why this attraction occurs. This also contributes to why sucrose is highly soluble in H2O.
