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Lecture

chapter2.pdf

10 Pages
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Department
Chemistry
Course Code
CHEM 281
Professor
John Carran

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Description
Hydrocarbons • hydrocarbons are compounds that contain only carbon and hydrogen atoms ◦ alkanes: hydrocarbons that do not have multiple bonds between carbon atoms ▪ e.g.: pentane, cyclohexane ◦ alkenes: contain at least one carbon–carbon double bond ▪ e.g.: propene, cyclohexene ◦ alkynes: contain at least one carbon–carbon triple bond ▪ e.g.: 1-pentyne, ethyne, 2-pentyne ◦ aromatic compound: contain a special type of ring, the most common example of which is a benzene ring benzylic protons: slightly acidic Smell: benzene:carcinogenic whiteout → extremely stable, hard formetabolic enzymes to metabolize → tries to make more polarto excrete → non polar:likely to dissolve in fatty tissues and stay there → also flat so can “intercollate” between helices ofDNA and disupt DNA replication -carninogenic - easier to polarize: if remove a proton and has multiple resonance forms:db between methyl and protons on toluene after proton negative charged moved around ring removed: protons on C2 vinyllic Alkanes • the primary sources of alkanes are natural gas and petroleum • the smaller alkanes (methane through butane) are gases under ambient conditions • methane is the principal component of natural gas → add thiols for smell (rotten eggs) • higher molecular weight alkanes are obtained largely by refining petroleum Alkenes • ethene and propene, the two simplest alkenes, are among the most important industrial chemicals produced in the United States • ethene (aka ethylene) is used as a starting material for the synthesis of many industrial compounds, including ethanol, ethylene oxide (cyclic ether) , ethanal, and the polymer polyethylene • propene is the important starting material for acetone, cumene and polypropylene (repeating units of propene) • examples of naturally occurring alkenes Alkynes • the simplest alkyne is ethyne (also called acetylene) ← ethyne • examples of naturally occuring alkynes terminal alkynes: acidic Benzene • all C C bond lengths are the same (1.39Å) ◦ compare with C–C single bond 1.54 Å, C=C double bond 1.34 Å • extra stabilization due to resonance → aromatic 3 Dimensional structure of benzene • planar structure 2 • all carbons sp hybridized • the lobes of each π orbital above and below the ring overlap with the lobes of π orbitals on the atoms to either side of it • the six electrons associated with these π orbitals (one electron from each orbital) are delocalized about all six carbon atoms of the ring Polar Covalent Bonds • lithium fluoride has an ionic bond • methane has a covalent bond: the electrons are shared equally between the carbon atoms Electronegativity (EN) • the intrinsic ability of an atom to attract the shared electrons in a covalent bond • electronegativities are based on an arbitrary scale, with F the most electronegative (EN = 4.0) and Cs the least (EN = 0.7) Polar and Nonpolar Molecules • dipole moments expressed in debyes (D), where 1D=3.336x10 -30coulomb meter (C●m) in SI → vector quantities • molecules containing polar bonds are not necessarily polar as a whole, for example • dipole moment of some compounds Strong base: use to remove polar protic → H attached proton from terminal alkyne nitromethane + - K NH :2use NH as s3lvent → if reacts w/ reactant 2NH ) all it does it creates more reactant (NH3→ NH )2 Dipole moments in alkanes Physical properties of some cis-trans Functional Groups → adding carbons decreases polarity wiggly lines = disconnection The general formula for an alkane is R-H Iso (isopropyl) Tert (tert-but.) T-but Sec ( Phenyl and Benzyl Groups • phenyl group • benzyl group Benzyle protons: acidic b/c of resonance tert butyl iodide Alkyl Halides or Haloalkanes + OR δ - δ+ - dimethylethyliodide • R-X (X=F, Cl, Br, I) δ δ • examples: Most reactive: easiest for Isopropyl bromide nucleophile to attack (space) Alcohols • R-OH • examples: • alcohols may be viewed structurally in two ways: ◦ as hydroxyl derivatives of alkanes Always wear gloves: extremely ◦ as alkyl derivative of waters acidic When remove proton from phenol: phenoxide → can draw many resonance structures for phenoxide → more stable, more favoured reactant in eqm → such strong acid: can burn skin Diethyl ether -used for lots ofsolvent b/c generally Ethers Tetra hydro furam unreactive (THF) -however does react w/ acid • R-O-R -used as solvent for acids • examples Amines • R-NH 2 -hyperconjugation:alkyl groups can release electron density towards a centre -reactivity:balance between hyperconjugation and available space Pyridine:smells like -lone pair ofelectron becomes rottenfish more negative Aldehydes and Ketones • Aldehydes and ketones have a trigonal planar arrangement of groups around the carbonyl carbon atom → carbonyl compounds (C=O) δ- formaldehyde + δ in presence ofacid:protonation ofoxygen → lone pairs
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