Newsgroups: alt.drugs From: [an 58264] at [anon.penet.fi] (Dalamar) Date: Wed, 18 May 1994 22:48:44 UTC Subject: CHEMISTRY: Polarity. Hi, After reading some of the posts on polarity and the differences between a polar and non-polar solvent, I sat down and tried to explain it in simple terms. I soon realised this isn't as easy as it sounds so the resulting text is quite a bit longer than i planned and strays now and again to introduce new concepts. Post your thoughts if you can make it through it all. If you find it ok and want to see more then i will go back and explain bonding, shapes of molecules, acids and bases and nomenclature, as well as the principle methods of purification etc. I feel that at least some background knowledge when doing a reaction can be greatly rewarding, especially when something goes wrong and you have to figure out what ! Solvents and Polarity You will need : A copy of the periodic table ! Look at the symbols for the elements in your periodic table. When written down like this each of the symbols you see can be taken to represent an ATOM of that element. Each atom is capable of joining or 'bonding' to other atoms to form _molecules_. You can think of the atoms as having a certain number of arms which it uses to form links. The proper term for the number of bonds an atom can form is its _valency_. For example hydrogen has a valency of 1 and thus can form only one bond, either to an atom of a different element or to another H atom. An example here might help.... Element: Hydrogen Symbol: H Valency: 1 Element: Oxygen Symbol: O Valency: 2 Now, most atoms don't like to be isolated and will take every opportunity to use up their valency and form links with other atoms, up to the maximum allowed for that element - in our above examples this is a mere one for hydrogen and two for oxygen. If you have a cylinder of 'hydrogen' gas, what is actually in the cylinder are millions of molecules of hydrogen made up of two atoms of hydrogen linked by a chemical bond, or arm in arm if you like. Each atom of H can only form one bond so we end up with the molecules made up of pairs of atoms. Hydrogen is thus a _diatomic_ molecule. This can be represented by H-H and written in shorthand form as H2 (the 2 should be subscript). When we turn to oxygen we see that its atoms are capable of forming two bonds. It has a valency of two and just like hydrogen it can bond to itself or to atoms of other elements. Combining H and O we can envisage : 1. H-O-H 2. H-O-O-H The first is probably the most familiar to you - H2O or water! The second is known as hydrogen peroxide. Examine the structures and see how each H atom forms only one bond and each O atom forms only two. Carbon has a valency of four, that is to say it can form 4 bonds to other atoms, be they other carbon atoms, oxygen atoms or hydrogen atoms etc. Carbon has a special ability - It can CATENTATE its atoms. This term means that carbon atoms are capable of linking together to form long chains containing many carbon atoms, numbering from 2 to thousands. Just using carbon, hydrogen and oxygen as our building blocks it is possible to link them together in different numbers and different ways which create hundreds of thousands of _compounds_. Some may have similair properties to each other - some radically different. Let's take an example - something you are all familiar with - alcohol. The alcohol found in beers etc is known as ethanol and has the molecular formula C2H6O. Its _structural formula_, which shows you which atoms are connected to which and in which order, is : H H | | H-C-C-O-H | | H H ETHANOL - STRUCTURAL FORMULA Counting the number of bonds which each atom forms you can see each carbon forms 4, oxygen 2, and all hydrogens 1. Another compound comprising only C, H and O is dimethyl ether: H H | | H-C-O-C-H | | H H Notice there is no O-H bond in this compound, instead the oxygen binds to 2 carbon atoms. Dimethyl ether has different properties to ethanol - it is very volatile compared to ethanol and drinking it probably wouldn't do you much good ! However..count the number of atoms of each element present in both ethanol and methanol ie look at the above molecular structures and write down the molecular formula again from this. Then compare them. They are the same ! How can this be ? It should be obvious from this example that a MOLECULAR FORMULA only shows the number of each kind of atom contained in a molecule of our compound. It does NOT tell us anything about the ORDER in which the atoms are connected. As the complexity of the m.formula increases so does the number of possible structures we can write for it. Each different structure is known as a _structural isomer_, in effect they are all structural isomers of each other as each shares the same molecular formula. The bonds which we represent as lines actually comprise of pairs of electrons. You can think of the pair of electrons as a sort of glue - they sit between the two 'bound' atoms and hold them together. The two atoms effectively share the pair of electrons but, as usual when it comes to sharing, sometimes this is not always equally. The elements on the far right of the periodic table have a real love for electrons and pull them hard when competing for them. Fluorine, the very top right element is the T.Rex of the elements in these regards. This property of 'electron pulling' is known as _electronegativity_ and it can be measured. A common scale used is the Pauling scale. Here fluorine is assigned the highest value of 4, with hydrogen around the 2.1 mark. Anything with an electronegativity of less than 2.1 doesnt have much pulling power and in fact these atoms are sometimes known as _electropositive_ because they compete so poorly for electrons. The lower the value the more 'electropositive' the atom ie electrons are given up more easily. Across rows in the table electronegativity increases from element to element. So oxygen for example is more electronegative than nitrogen, but fluorine in turn is more electronegative than oxygen. Down the colums of the table electronegativity falls, so that chlorine is less electronegative than fluorine and bromine in turn is less electronegative than chlorine. Accompanied by this decrease in electronegativity down the group is an increase in the size of the atoms. When two atoms of the same electronegativity are bound to each other, as in H2 or F2 then each atom competes equally for the electron pair and thus an equal share is achieved ie you can visualise the pair as sitting exactly half way between the atoms. However, with atoms of differing electronegativity the pair of electrons is pulled closer to the _more electronegative atom_. The more electronegative atom thus aquires a slight negative charge (electrons are negatively charged) and the less electronegative atom aquires a slight positive charge. To distinguish these slight charges from full blown ones the greek symbol delta is used eg d+ represents a partial positive charge. Such a bond is said to be _polarised_ ie the negative charge is polarised in the direction of the more EN atom. A few examples will illustrate this point. 1. H-F The fluorine is the more EN of the two and thus gets the bigger share of the electrons and aquires a partial negative charge d-. The H on the other hand is left with a slight deficiency of electrons and thus aquires a d+ charge. 2. H-O-H Here oxygen is the more EN of the two atom types present. Thus each H has a partial positive charge and the oxygen a partial negative one. The above explained how bonds between atoms can be _polar_, but what about the molecule as a whole ?? Why is CCl4 (carbon tetrachloride) non polar as a whole although each C-Cl bond is indeed polarised with partial negative charge on Cl ? The answer to this question is that the molecule is not flat. Each pair of electrons which constitute the 4 single bonds will repel each other as they are areas of negative charge. This repulsion is much like trying to put the two north pole ends of a magnet together - they repel because they are the same pole, however a north pole will attract a south pole, just like a positive charge will attract a negative one. The electron pairs will thus get as far away as possible in order to minimise the repulsion they mutually feel. To do this each bond points itself into the corners of a regular tetrahedron, the Cl-C-Cl bond angle is around 109 degrees, not 90 degrees as the written down structure suggests. In this arrangement repulsive forces are minimised. The resulting symmetry of the molecule results in a sort of overall cancellation of polarisation. This effect is easier to see with a diatomic molecule such as BeCl2 (beryllium chloride). This molecule is linear ie the Cl-Be-Cl bond angle is 180 degrees. Each bond is polarised in the direction of the Cl which thus aquires a partial negative charge d-. The Be atom is thus left partially postitively charged. If we represent the direction of the polarisation as an arrow, and the length of the arrow represents the _magnitude_ of the effect then in BeCl2 we get two arrows of _equal_ length pointing in opposite directions. These arrows are referred to as vectors and have the property of being additive. A vector can be used to represent any property which has both magnitude and direction. Properties which only have magnitude are known as scalars. The effect of having two vectors of equal length (magnitude) pointing in exactly opposite directions is overall cancellation. It's like a tug of war between two teams that are exactly equal in strength - they are pulling in opposite directions so the rope stays where it is. However if one team is stronger then the rope will move in their direction, the force on the rope now being positive in their direction and equal to the difference in the two teams strength. In BeCl2 we have such a case : Cl-Be-Cl <~~~~~~ ~~~~~~> Polarisation is equal but in opposite directions. Overall effect is total cancellation and the molecule as a whole is non-polar. In CCl4 the 4 polarisation vectors point to the corners of a regular tetrahedron and are equal in length (magnitude), the overall effect is cancellation and zero net polarisation. Compare this to chloroform for example where the dominating EN of oxygen polarises the O-H bond more than the C-O bond. Here the vectors are not equal and the molecule as a whole is polar. Compare this to CHCl3 (chloroform) where one of the Cls has been replaced by an atom of much less EN. The polarisation vectors due to C-Cl are equal in magnitude but not direction - remember they point to the corners of a tetrahedron. The C-H bond polarisation vector is much smaller and because carbon is more EN than hydrogen the vector points in the opposite direction towards the centre face of the tetrahedron which touches all 3 Cl atoms. Trying to draw this in 2D doesn't give the proper picture but it gives you an idea. Cl | <~~~~~~~~~~~ Cl-C-H <~~~ small vector along CH bond. Overall vector due to | C-Cl polarisations Cl <~~~~~~~~~~~~~~~~ Total vector for entire molecule. So chloroform is a more polar solvent than CCl4. Similar arguments, based on symmetry and polarity of bonds, can be applied to other molecules to determine their overall polarity. Dalamar. ------------------------------------------------------------------------- To find out more about the anon service, send mail to [h--p] at [anon.penet.fi.] Due to the double-blind, any mail replies to this message will be anonymized, and an anonymous id will be allocated automatically. You have been warned. Please report any problems, inappropriate use etc. to [a--m--n] at [anon.penet.fi.]