l03 - CH 203 O Dipoles R G A N I C C H E M I S T R Y I In a...

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Dipoles © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Dipoles In a previous lecture we examined how atoms hybridize when forming bonds. In this lecture we address the question: How do we know that hybridization the- ory is correct? Electronegativity One of the most important attributes of an atom is its electronegativity (symbolized by the Greek letter chi ! ), which is defined as that atom’s abil- ity to attract electrons. Robert Mullikan (Nobel Prize, 1966) proposed that electronegativity ! be calculated by the formula ! = (0.00197) × (IP 1 – EA) + 0.19 where IP 1 is an atom’s first ionization potential and EA is that atom’s elec- tron affinity, both in units of kilojoule per mole; ! has no units. A large value of IP 1 means that an atom gives up an electron reluctantly: it is good at holding on to the electrons it has. A negative value of EA means that an atom captures an electron readily. Thus, the most electronegative at- oms should have a large value of IP 1 and a negative value of EA; conversely, the least electronegative atoms should have a small value of IP 1 and a posi- tive value of EA. Problem Calculate the Mullikan electronegativity of fluorine (IP 1 = 1681 kJ/mol, EA = –333 kJ/mol) and of carbon (IP 1 = 1086 kJ/mol, EA = –121 kJ/mol). Answer For fluorine ! = 4.16; for carbon ! = 2.57. Fluorine is much more elec- tronegative than carbon. This result could have been anticipated given that fluorine has both a larger value of IP and a more negative value of EA. There are two fairly reliable periodic-table trends in electronegativity: electronegativity increases from left to right across a row and increases up a column. An important exception to these general trends is that the electro- negativity of hydrogen ( ! = 2.20) actually falls between that of boron ( ! = 2.04) and carbon ( ! = 2.55) even though hydrogen appears far to the left on most periodic tables. We should also note that, among the elements most im-
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Dipoles © Bruno I. Rubio 2 portant to organic chemistry, N, O, F, S, Cl, Br and I are more electronega- tive than carbon. Bond dipoles Because electronegativity is the ability to attract electrons, whenever two atoms of different electronegativity bond, electrons flow from the less elec- tronegative to the more electronegative atom. In ionic bonds the flow of electrons is extreme: we characterize ionic bonds as the complete transfer of electrons from the much less electronegative metal to the much more electro- negative non-metal. The flow of electrons from the less to the more electro- negative atom also takes place in covalent bonds, albeit to a lesser extent than in ionic bonds. Let’s consider a covalent bond between atoms of different electronegativity.
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l03 - CH 203 O Dipoles R G A N I C C H E M I S T R Y I In a...

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