Sp-orbitals

All four sp orbitals are of equal energy Therefore according to Hund s rule (Sec tion 1 1) the four valence electrons of carbon are distributed equally among them making four half filled orbitals available for bonding... 

The axes of the sp orbitals point toward the corners of a tetrahedron Therefore sp hybridization of carbon is consistent with the tetrahedral structure of methane Each C—H bond is a ct bond m which a half filled Is orbital of hydrogen over laps with a half filled sp orbital of carbon along a line drawn between them... 

FIGURE 2 8 sp Hybridization (a) Electron configuration of carbon in its most stable state (b) Mixing the s orbital with the three p orbitals generates four sp hybrid orbitals The four sp hybrid orbitals are of equal energy therefore the four valence electrons are distributed evenly among them The axes of the four sp orbitals are directed toward the corners of a tetrahedron... 

FIGURE 2 9 Each half filled sp orbital overlaps with a half filled hydrogen Is or bital along a line between them giving a tetrahedral arrangement of four ct bonds Only the major lobe of each sp orbital is shown Each orbital contains a smaller back lobe which has been omitted for clarity... 

FIGURE 2 10 The C—C ct bond in ethane pictured as an overlap of a half filled sp orbital of one carbon with a half filled sp hybrid orbital of the other... 

Each carbon of ethylene uses two of its sp hybrid orbitals to form ct bonds to two hydrogen atoms as illustrated m the first part of Figure 2 17 The remaining sp orbitals one on each carbon overlap along the mternuclear axis to give a ct bond connecting the two carbons... 

Because each carbon m acetylene is bonded to two other atoms the orbital hybridization model requires each carbon to have two equivalent orbitals available for CT bonds as outlined m Figure 2 19 According to this model the carbon 2s orbital and one of Its 2p orbitals combine to generate two sp hybrid orbitals each of which has 50% s character and 50% p character These two sp orbitals share a common axis but their major lobes are oriented at an angle of 180° to each other Two of the original 2p orbitals remain unhybridized... 

Section 2 20 Carbon is sp hybridized in ethylene and the double bond has a ct com ponent and a rr component The sp hybridization state is derived by mix mg the 2s and two of the three 2p orbitals Three equivalent sp orbitals result and their axes are coplanar Overlap of an sp orbital of one car bon with an sp orbital of another produces a ct bond between them Each carbon still has one unhybridized p orbital available for bonding and side by side overlap of the p orbitals of adjacent carbons gives a rr bond between them... 

Section 2 21 Carbon is sp hybridized m acetylene and the triple bond is of the ct + Tt + Tt type The 2s orbital and one of the 2p orbitals combine to give two equivalent sp orbitals that have their axes m a straight line A ct bond between the two carbons is supplemented by two tr bonds formed by overlap of the remaining half filled p orbitals... 

FIGURE 4 19 Bonding in methyl radical (a) If the structure of the CH3 radical IS planar then carbon is sp hybridized with an unpaired electron in 2p orbital (b) If CH3 IS pyramidal then car bon IS sp hybridized with an electron in sp orbital Model (a) IS more consistent with experimental observa tions... 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

All of these trends can be accommodated by the orbital hybridization model The bond angles are characteristic for the sp sp and sp hybridization states of carbon and don t require additional comment The bond distances bond strengths and acidities are related to the s character m the orbitals used for bonding s Character is a simple concept being nothing more than the percentage of the hybrid orbital contributed by an s orbital Thus an sp orbital has one quarter s character and three quarters p an sp orbital has one third s and two thirds p and an sp orbital one half s and one half p We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and p contributors... 

FIGURE 1116 (a) Pyridine has six tt electrons plus an unshared pair in a nitrogen sp orbital (b) Pyrrole has six tt electrons (c) Furan has six tt electrons plus an unshared pair in an oxygen sp orbital which is perpendicular to the tt system and does not interact with it... 

The sp hybrid state of nitrogen is just like that of carbon except nitrogen has one more electron Each N—H bond in NH3 involves overlap of an sp hybrid orbital of N with a li orbital of hydrogen The unshared pair of NH3 occupies an sp orbital... 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Figure 2.3 The shapes of orbitals for the s electron pair, the three pairs of p electrons with obitals mutally at right angles, and the sp orbitals which have the major lobes pointing towards the apices of a regular tetrahedron.

Here, the bonding between carbon atoms is briefly reviewed fuller accounts can be found in many standard chemistry textbooks, e.g., [1]. The carbon atom [ground state electronic configuration (ls )(2s 2px2py)] can form sp sp and sp hybrid bonds as a result of promotion and hybridisation. There are four equivalent 2sp hybrid orbitals that are tetrahedrally oriented about the carbon atom and can form four equivalent tetrahedral a bonds by overlap with orbitals of other atoms. An example is the molecule ethane, CjH, where a Csp -Csp (or C-C) a bond is formed between two C atoms by overlap of sp orbitals, and three Csp -Hls a bonds are formed on each C atom. Fig. 1, Al. 

The sp orbitals are equivalent, coplanar and oriented at 120° to each other and form a bonds by overlap with orbitals of neighbouring atoms, as in the molecule ethene, CjH, Fig. 1, A2. The remaining p orbital on each C atom forms a n bond by overlap with the p orbital from the neighbouring C atom the bonds formed between two C atoms in this way are represented as Csp =Csp, or simply as C=C. 

Fig. 1.3. Valenee bond stmetural representation of methane resulting from overlap of H li orbitals with four equivalent sp orbitals of earbon. ...

There have been several representations of the bonding in benzyne. The one most generally used pictures benzyne as being similar to benzene but with an additional weak bond in the plane of the ring, formed by overlap of the two sp orbitals. Comparison of the NMR characteristies with MO ealeulations indieate that the eonjugation is maintained and that benzyne is a strained but aromatie moleeule. ... 

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