UConn CHEM 2443 - Chapter 5: Functional Groups

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Chapter 5: Functional GroupsAlkenes- Alkenes: hydrocarbons that contain @ least 1 pi bond b/w C’so 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 propertieso 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 atomso 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 bondo 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 thesigma 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 bondo 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 bondso 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 CChapter 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 lineso 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 equalfor both lobes of the pi bond, not part on the top and part on the bottomo B/c of this, less electron density is concentrated b/w the 2 nuclei than w/ a sigma bondo 2 electrons are located in a pi bondo The overlap of these 2 electrons is not as efficient as if the electron density were concentrated on a line b/wthe nuclei, so the pi bond is weaker than the sigma bondo The pi bond constitutes a second bond to C, so the 2 C atoms are joined by a double bond- Nomenclature:o Suffix for alkenes: “—ene”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 bondby 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 Co 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 bondo 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 ito 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 orbitalo Triple Bond b/w C’s 1 sigma bond and 2 pi bondso 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 ofthe triple bond Formation of the sigma bond leaves 2 unused p-orbitals on each Co The unused p-orbitals overlap in a sideways manner to form 2 pi bondso 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 bondso The remaining 2 electrons of the p-orbitals are used toform the 2 pi bonds The p-orbitals on the atom are mutually perpendicular, so the 2 pi bonds formed byChapter 5: Functional Groupsoverlap of those orbitals must be perpendicular to each other when 2 pi bonds are b/w adjacent C atomso 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 atomso A single covalent bond is formed using one 2p-orbitaland one 2s-orbital and 2 p-orbitals are not usedo In sp hybridization, b/c the 2 pi bonds are mutually perpendicular, they essentially react independently of one another. It’s possible for the electrons in one pi bond to interact w/ another molecule w/o disturbing the electrons in the second pi bond.- Alkyne: a hydrocarbon that contains 2 pi bonds to form a triple bonded Co Rxns lead to diff products than alkanes and alkeneso Form C chains and branched C chainso Stable cyclic compounds must be larger to accommodate the linear nature of triple bonds C’s B/c if it was too small there would

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UConn CHEM 2443 - Chapter 5: Functional Groups

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