Chapter 5 Functional Groups Alkenes Alkenes hydrocarbons that contain least 1 pi bond b w C s o The longest linear chain contains a C C unit the functional group Each C atom of the C C unit will have 4 bonds but only 3 of the bonds are sigma bonds and the fourth is a pi bond o CnH2n n can t be 1 Functional Group collection of atoms w certain chemical properties o Diff compounds that contain the same functional group will have specific diff physical properties but will have many chem properties that are identical Sigma bonds of alkenes are formed using sp2 hybrid molecular orbitals o One of the 2s electrons of C is promoted to form the hybrid but only 3 hybrids of the sp2 hybrid orbitals are used to form 3 sigma bonds to other atoms o Leaves an unused orbital on each C and each orbital has an extra electron The unused electrons are shared b w the 2 atoms to form another covalent bond o The sigma bonds are due to overlap of the sp2 hybrid orbitals and the overall geometry is trigonal planar about each C b c it s the lowest E arrangement 120 degrees The extra molecular orbital on each C after formation of the sigma bond using the sp2 hybrid orbitals is perpendicular on the plane of atoms o After formation of the sigma bonds the only way adjacent sp2 hybridized C atoms can share electron density is by sideways overlap to form a pi bond Less electron density is shared by the adjacent C atoms in the pi bond than in the sigma bond so the pi bond is weaker than the sigma bond o In a sigma bond the shared electron density is on a line b w 2 nuclei whereas in the pi bond the shared electron density is on a line b w parallel orbitals Result of the sp2 hybridization is 2 bonds b w the C atoms Weaker pi bond and a stronger sigma bond 3 atoms surround C as they approach to form 3 sigma bonds o All 3 atoms are in one plane x y z o They can overlap only w the 2px and 2py orbitals but NOT w the 2pz orbital o These atoms can also overlap w the 2s orbital but cannot approach closely enough to overlap the 1s orbital of C Chapter 5 Functional Groups Each sigma bond is formed using two 2p orbitals and one 2s orbital of C Aka sp2 hybridization o The third 2p orbital is perpendicular to the plane formed by the 3 atoms connected to C and is NOT used for bonding If 2 of the sp2 hybrid orbitals on each C just described are brought 2gether o One sp2 hybrid orbital on each C will overlap to form a sigma bond most of the electron density is concentrated on a line b w the nuclei Represented by the usual lines wedges and dashed lines o The other 2 sp2 hybrid orbitals on each C are used to form sigma bonds to other atoms represented by R R refers to any alkyl substituent so it s any C group These 2 orbitals are close enough that they can share electron density but not along a line b w nuclei The only way the 2 p orbitals can overlap share electron density is by sideways overlap to form a new type of bond called a pi bond In a pi bond the probability of finding electron density is equal for both lobes of the pi bond not part on the top and part on the bottom o B c of this less electron density is concentrated b w the 2 nuclei than w a sigma bond 2 electrons are located in a pi bond o o The overlap of these 2 electrons is not as efficient as if the electron density were concentrated on a line b w the nuclei so the pi bond is weaker than the sigma bond o The pi bond constitutes a second bond to C so the 2 C atoms are joined by a double bond Suffix for alkenes ene Nomenclature o o More than one molecule can have the same name such as octane so to distinguish them the name must specifically designate the position of the pi bond by numbering the chain so that the first C of the pi bond receives the lowest possible Chapter 5 Functional Groups There are also cyclic alkenes o The parent name for cyclic alkenes is based on the of C atoms in the ring derived from the analogous linear alkene but the prefix cyclo is added to the name Give subs the lowest numbering possible In an alkene 3 atoms surround each indiv C o One of the 3 atoms is the other C connected by a sigma bond and a pi bond The 2 C atoms in a C C unit should be held more closely together by 2 covalent bonds than w in a C C unit w only one covalent bond o So a double bond is shorter than a single bond which means that the internuclear distance b w the 2 C atoms is shorter Every C has the max of H atoms attached to it o The presence of a C C unit means that there are 2 fewer H atoms when compared to an alkane The formula is the same as a cycloalkane so they are isomers Alkynes sp Hybridization o sp hybrid orbitals are made up of one s orbital and one p orbital o Triple Bond b w C s 1 sigma bond and 2 pi bonds o In a triple bond each sp hybridized C will use one hybrid orbital to form a sigma bond to another atom and one hybrid to form a sigma bond to the other C of the triple bond Formation of the sigma bond leaves 2 unused p orbitals on each C o The unused p orbitals overlap in a sideways manner to form 2 pi bonds o Each C will promote the 2s electrons to form the sp hybrid orbital but only 2 of the resulting hybrids are used to form sigma bonds o The remaining 2 electrons of the p orbitals are used to form the 2 pi bonds The p orbitals on the atom are mutually perpendicular so the 2 pi bonds formed by Chapter 5 Functional Groups overlap of those orbitals must be perpendicular to each other when 2 pi bonds are b w adjacent C atoms o Leads to a linear relationship and a presence of 2 pi bonds B c it s the lowest E arrangement of 2 atoms as they approach C to form sigma bonds In alkenes the electron density is concentrated above and below the plane of the atoms o o A single covalent bond is formed using one 2p orbital and one 2s orbital and 2 p orbitals are not used In sp hybridization b c the 2 pi bonds are mutually perpendicular they essentially react independently of one another It s possible for the …