Unshared

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

It will always be true that a nitrogen with four covalent bonds has a formal charge of + 1 (A nitrogen with four co valent bonds cannot have unshared pairs because of the octet rule)... 

It will always be true that an oxygen with two covalent bonds and two unshared pairs has no formal charge... 

The green oxygen m Figure 1 5 owns three unshared pairs (six electrons) and shares two electrons with nitrogen to give it an electron count of seven This is one more than the number of electrons m the valence shell of an oxygen atom and so its formal charge is —1... 

FIGURE 1 5 Counting electrons in nitnc acid The electron count of each atom is equal to half the number of electrons it shares in covalent bonds plus the number of electrons in its own unshared pairs... 

If one or more atoms have fewer than eight elec trons use unshared pairs on an adjacent atom to form a double (or triple) bond to complete the octet... 

The structure shown is the best (most stable) Lewis structure for methyl nitrite All atoms except hydro gen have eight electrons (shared + unshared) in their valence shell... 

Organic chemists have devised a number of shortcuts to speed the writing of structural formulas Sometimes we leave out unshared electron pairs but only when we are sure enough m our ability to count electrons to know when they are present and when they re not We ve already mentioned representing covalent bonds by dashes In condensed structural formulas we leave out some many or all of the covalent bonds and use sub scripts to indicate the number of identical groups attached to a particular atom These successive levels of simplification are illustrated as shown for isopropyl alcohol ( rub bmg alcohol )... 

As you practice you will begin to remember patterns of electron distribution A neutral oxygen with two bonds has two unshared electron pairs A neutral nitro gen with three bonds has one unshared pair ... 

When writing a Lewis structure we restrict a molecule s electrons to certain well defined locations either linking two atoms by a covalent bond or as unshared electrons on a sm gle atom Sometimes more than one Lewis structure can be written for a molecule espe cially those that contain multiple bonds An example often cited m introductory chem istry courses is ozone (O3) Ozone occurs naturally m large quantities m the upper atmosphere where it screens the surface of the earth from much of the sun s ultraviolet rays Were it not for this ozone layer most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun The following Lewis structure for ozone satisfies fhe ocfef rule all fhree oxygens have eighf elecfrons m fheir valence shell... 

The tetrahedral geometry of methane is often explained with the valence shell electron pair repulsion (VSEPR) model The VSEPR model rests on the idea that an electron pair either a bonded pair or an unshared pair associated with a particular atom will be as far away from the atom s other electron pairs as possible Thus a tetrahedral geomehy permits the four bonds of methane to be maximally separated and is charac terized by H—C—H angles of 109 5° a value referred to as the tetrahedral angle... 

The H—O—H angle m water (105°) and the H—N—H angles m ammonia (107°) are slightly smaller than the tetrahedral angle These bond angle contractions are easily accommodated by VSEPR by reasoning that electron pairs m bonds take up less space than an unshared pair The electron pair m a covalent bond feels the attractive force of... 

Water (H2O) 105 H V. 0— / Oxygen has two bonded pairs + two unshared pairs Tetrahedral Bent <4... 

Ammonia (NH3) 107 H / Nitrogen has three bonded pairs + one unshared pair Tetrahedral Trigonal pyramidal ... 

Bonded pair bonded pair > Unshared pair bonded pair > Unshared pair unshared pair Least repulsive Most repulsive... 

In the equation shown the base uses an unshared pair of electrons to remove a proton from an acid The base is converted to its conjugate acid, and the acid is converted to... 

Writing the equation in the usual way directs too much attention to the atoms and not enough to the electrons We can remedy that by deleting any spec tator ions and by showing the unshared electron pairs and covalent bonds that are made and broken Both sodium hydroxide and sodium fluoride are com pletely ionized in water therefore Na" which ap pears on both sides of the equation is a spectator ion Hydrogen fluoride is a weak acid and exists as undissociated HF molecules in water... 

Now use curved arrows to track electron flow Move electrons from sites of high electron density (unshared pairs negative charge) toward sites of lower electron density... 

One of the unshared pairs of the hydroxide oxygen is used to form a covalent bond to the positively polar ized proton of hydrogen fluoride The covalent bond betwen H and F in hydrogen fluoride breaks with the pair of electrons in this bond becoming an unshared pair of fluoride ion... 

An unshared pair of electrons from the Lewis base is used to form a covalent bond between the Lewis acid and the Lewis base The Lewis acid and the Lewis base are shown as ions m the equation but they need not be If both are neutral molecules the analogous equation becomes... 

The carbon atom m bromomethane can accept an electron pair if its covalent bond with bromine breaks with both electrons m that bond becoming an unshared pair of bromide ion Thus bromomethane acts as a Lewis acid m this reaction... 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

Lewis bases use an unshared pair to form a bond to some other atom and are also referred to as nucleophiles ( nucleus seekers ) Conversely Lewis acids are elec trophiles ( electron seekers ) We will use these terms hundreds of times throughout the remaining chapters... 

Expand the following structural representations so as to more clearly show all the atoms and any unshared electron pairs... 

Wnte an equation for the Brpnsted acid-base reaction that occurs when each of the fol lowing acids reacts with water Show all unshared electron pairs and formal charges and use curved arrows to track electron movement... 

Describe the bonding in ammonia assuming sp hybridization of nitrogen In what kind of orbital is the unshared pair What orbital overlaps are involved in the N—H bonds , ... 

Write the Lewis structure and include unshared electron pairs... 

The Lewis bases that react with electrophiles are called nucleophiles ( nucleus seek ers ) They have an unshared electron pair that they can use m covalent bond formation The nucleophile m Step 3 of Figure 4 6 is chloride ion... 

As shown m Figure 4 11 the crucial electronic interaction is between an unshared elec tron pair of Cl and the vacant 2p orbital of the positively charged carbon of (CH3)3C ... 

The absolute configuration at sulfur is specified by fhe Cahn-Ingold-Prelog mefhod wifh fhe provision fhaf fhe unshared elecfron parr is considered fo be fhe lowesf ranking subsfifuenf... 

---