Electrostatic potential mapping

The electrostatic potential map of hydrogen fluoride (HF) was shown in the preceding section and IS repeated here Compare it to the electrostatic po tential map of lithium hydride (LiH)... 

Thus electrostatic potential maps can give an exag gerated picture of the charge distribution when the entire palette is used In most cases that won t mat ter to us inasmuch as we are mostly concerned with the distribution within a single molecule In those few cases where we want to compare trends in a se ries of molecules we II use a common scale and will point that out... 

FIGURE 1 6 Molecular models of methane (CH4) (a) Framework (tube) models show the bonds connecting the atoms but not the atoms themselves (b) Ball and stick (ball and spoke) models show the atoms as balls and the bonds as rods (c) Space filling models portray overall molecular size the radius of each sphere approximates the van der Waals radius of the atom (d) An electrostatic potential map of methane... 

The greater positive character hence the increased acidity of the O—H proton of 2 2 2 tnfluoroethanol can be seen m the electrostatic potential maps displayed m Figure 1 8 Structural effects such as this that are transmitted through bonds are called indue tive effects A substituent induces a polarization m the bonds between it and some remote site A similar inductive effect is evident when comparing acetic acid and its trifluoro derivative Trifluoroacetic acid is more than 4 units stronger than acetic acid... 

FIGURE 1 8 Electrostatic potential maps of ethanol and 2 2 2 tnfluoroethanol As indi cated by the more blue less green color in the region near the OH proton in 2 2 2 trifluoro ethanol this proton bears a greater degree of positive charge and is more acidic than the OH proton in ethanol... 

Examine the molecular models of H2 HE CH4 CH3F and CF4 Find the calculated dipole moment of each compound and examine their electrostatic potential maps... 

Figure 2 4 uses electrostatic potential maps to show this build up of electron den sity m the region between two hydrogen atoms as they approach each other closely enough for their orbitals to overlap... 

The structure of ethylene and the orbital hybridization model for its double bond were presented m Section 2 20 and are briefly reviewed m Figure 5 1 Ethylene is planar each carbon is sp hybridized and the double bond is considered to have a a component and a TT component The ct component arises from overlap of sp hybrid orbitals along a line connecting the two carbons the tt component via a side by side overlap of two p orbitals Regions of high electron density attributed to the tt electrons appear above and below the plane of the molecule and are clearly evident m the electrostatic potential map Most of the reactions of ethylene and other alkenes involve these electrons... 

FIGURE 5 1 (a) The planar framework of u bonds in ethylene showing bond distances and angles (b) and (c) The p orbitals of two sp hybridized carbons overlap to produce a tt bond (d) The electrostatic potential map shows a region of high negative potential due to the tt elec trons above and below the plane of the atoms... 

Examine the electrostatic potential map of H3B THE (borane-tetrahydrofuran complex) on Learning By Modeling How does the electrostatic potential of the hydrogens bonded to boron dif fer from the potential of the hydrogens of the tetrahydrofuran ring" ... 

An sp hybridization model for the carbon-carbon triple bond was developed in Section 2 21 and is reviewed for acetylene in Figure 9 2 Figure 9 3 compares the electrostatic potential maps of ethylene and acetylene and shows how the second tr bond m acetylene causes a band of high electron density to encircle the molecule... 

FIGURE 113 (a) The framework of bonds shown in the tube model of benzene are cr bonds (b) Each carbon is sp hybridized and has a 2p orbital perpendicular to the cr framework Overlap of the 2p orbitals generates a tt system encompass mg the entire ring (c) Electrostatic potential map of benzene The red area in the center corresponds to the region above and below the plane of the ring where the tt electrons are concentrated... 

The electrostatic potential map of benzene (Figure 11 3c) shows regions of high electron density above and below the plane of the ring which is where we expect the most loosely held electrons (the rr electrons) to be In Chapter 12 we will see how this region of high electron density is responsible for the characteristic chemical reactivity of benzene and its relatives... 

The electrophile (E ) m this reaction is mtromum ion (0=N=0) The charge distn bution m mtromum ion is evident both m its Lewis structure and m the electrostatic potential map of Figure 12 2 There we see the complementary relationship between the electron poor region near nitrogen of NO, and the electron rich region associated with the TT electrons of benzene... 

FIGURE 12 8 Electrostatic potential map of propanoyl cation [(CH3CH2C=0) ] The region of greatest positive charge is associated with the carbon of the C=0 group... 

Figure 14 1 uses electrostatic potential maps to show how different the electron distri bution IS between methyl fluoride (CH3F) and methyllithium (CH3L1)... 

FIGURE 14 3 (a) The unshared electron pair occupies an sp hybridized orbital in dichlorocarbene There are no electrons in the unhybridized p orbital (b) An electrostatic potential map of dichlorocarbene shows negative charge is concentrated in the region of the unshared pair and positive charge above and below the carbon... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

FIGURE 15 7 Electrostatic potential maps of (a) methanol and (b) methanethiol The color scales were adjusted to be the same for both molecules to allow for direct comparison The development of charge is more pronounced in the region surrounding the —OH group in methanol than it is for the —SH group in methanethiol... 

Find the molecular model of 18 crown 6 (see Figure 16 2) on Learning By Modeling and examine its electrostatic potential map View the map in vanous modes (dots contours and as a transparent surface) Does 18 crown 6 have a dipole moment Are vicinal oxygens anti or gauche to one another"d... 

Find the model of dimethyl sulfoxide [(CH3)2S=0] on Learning By Modeling and exam me Its electrostatic potential map To which atom (S or O) would you expect a proton to bond d... 

Learning By Model ing includes models of formaldehyde (H2C=0) and its protonated form (H2C=0H ) Compare the two with respect to their electrostatic potential maps and the degree of positive charge at carbon... 

The graphic that opened this chapter IS an electrostatic potential map of the enol of acetone... 

Lone pair donation from the hydroxyl oxygen makes the carbonyl group less elec trophilic than that of an aldehyde or ketone The graphic that opened this chapter is an electrostatic potential map of formic acid that shows the most electron rich site to be the oxygen of the carbonyl group and the most electron poor one to be as expected the OH hydrogen... 

FIGURE 19 5 Electrostatic potential map of sodium stearate Most of the molecule is comprised of a nonpolar hydrocarbon chain (green) One end is very polar as indicated by the red and blue associated with the carboxylate and sodium ions respectively... 

You can examine the structure of methyl amine including its electrostatic potential map in more detail on Learning By Modeling... 

An orbital hybridization description of bonding m methylamme is shown m Figure 22 2 Nitrogen and carbon are both sp hybridized and are joined by a ct bond The unshared electron pair on nitrogen occupies an sp hybridized orbital This lone parr IS involved m reactions m which amines act as bases or nucleophiles The graphic that opened this chapter is an electrostatic potential map that clearly shows the concentration of electron density at nitrogen m methylamme... 

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