* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Organic Chemistry Structures of Organic Compounds
Elias James Corey wikipedia , lookup
Woodward–Hoffmann rules wikipedia , lookup
Ring-closing metathesis wikipedia , lookup
Organosulfur compounds wikipedia , lookup
Wolff rearrangement wikipedia , lookup
Asymmetric induction wikipedia , lookup
Hofmann–Löffler reaction wikipedia , lookup
Stille reaction wikipedia , lookup
Wolff–Kishner reduction wikipedia , lookup
George S. Hammond wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Tiffeneau–Demjanov rearrangement wikipedia , lookup
Baylis–Hillman reaction wikipedia , lookup
Ene reaction wikipedia , lookup
Petasis reaction wikipedia , lookup
Hydroformylation wikipedia , lookup
Nucleophilic acyl substitution wikipedia , lookup
Organic Chemistry You will need to buy a separate book “Organic Notes” Purchase from Mrs. Marston Why study Organic Chemistry? Organic chemistry is the chemistry generally practiced by living things. For biological efficiency, need readily available atoms and light atoms. The chemistry is primarily based on C, H, N and O; also some P, Cl, Br, S (and some transition elements in enzymes) At the moment ~ 1 x 107 organic compounds described in the literature either natural or unnatural Why is carbon so great? Its multiple bonding structures allow a great deal of complexity from a few different elements. Carbon can form stable C-X, C=X, C≡X bonds, X = C, O, and N. No other element can do this. Thus, many different structural possibilities exist. In the organic chemistry section, we are going to learn only 21 reactions and no new conceptual ideas. Structures of Organic Compounds Carbon is tetravalent - 4 covalent bonds. Carbon is usually positively charged 109.5o sp3 hybridized Polarity very weak very strong Bond δδC-Hδδ+ C-C δ+ C-OδC-Nδδ+ C-Clδδ+ Bond Energy (kJ/mol) 410 350 350 300 335 polar strongly polarized δ+ H-ClδδN-Hδ+ 430 400 Weak O-O I-I 140 140 Nomenclature Naming compounds – this is less important except for communication (it’s a pain, but less difficult than referring to “that thing with the thingy hanging off it.” Types of Compounds Bond Type Class E.g CH, CC Alkanes H3C-CH3 ethane CH, CC, CN Amine H3CH2CNH2 aminoethane CH, CC, CO Alcohol H3CCH2OH ethanol CH, CC, CO Ether H3CCH2OCH2CH3 ethyl ether CH, CC, CX X=Cl, Br, I Alkyl Halide BrCH2CH2Br 1,2-dibromoethane C-C, C=C, CH Alkene H2C=CH2 ethene C-C, CH, C=O Aldehyde O C H 3C H acetaldehyde C-C, CH, C=O O Ketone C H 3C CH3 (2-propanone) C-C, C≡C, CH Alkyne HC≡CH acetylene, ethyne C=C, CH, Aromatic benzene C=O, C-O, C-C, CH O Carboxylic Acid C H 3C OH acetic acid Alkane (CnH2n+2) Nomenclature: Structural Isomers Number of Carbons Root Number of Isomers 1 2 3 Meth Eth Prop 1 1 1 H2 C H 3C 4 But CH3 C H 3C H 5 6 7 . 10 . 40 H2 C CH3 2 Pent Hex Hept 3 5 9 Dec 75 CH3 H 3C 62,491,178,805,831 H3C -H → H3C-CH2-H → H3C-CH2-CH2 - H Take the previous analogue, replace H with CH2H to get higher homologue. 3D Structures of Molecules Depends on the available orbitals. For bonding, first look at the atomic orbitals: First, establish the Bonding no double / triple bonds H H H C e.g. CH4 H C H2 CH3 geometry for C (and all 2nd row elements) 4 atomic orbitals 1xs 3xp 2 rules (2s) spherically symmetric 2 px x axis 2 py y axis 2 pz z axis i) Hund’s rule - leave isoenergetic electrons unpaired ii) maximize electrostatic repulsion (i.e., separate electronic pairs as much as possible) The carbon in CH4 has 4 bonds (one to each H). We need to use all four atomic orbitals to make the 4 molecular orbitals (4 SIGMA σ orbitals). Mix 1 x 2s + 3 x 2p and get 4 x sp3 orbitals. In a tetrahedral compound, the 4 groups will be separated by about 109.5° - this is the normal geometry for carbon. H H H C 109.5o H Alkenes Use ethene as an example; each carbon has 3 bonds (1 x C, 2 x H). Need 3 atomic orbitals to give 3 molecular orbitals. 2s + 2px + 2py → 3 x sp3 σ orbitals. (Note no z-coordinates, just 3 substituents in the x-y plane). How to maximize repulsion? Separate by 120°. H H C H 120 o C H We have used 3 of the 4 electrons on carbon, the last electron is in a pz orbital these combine, on adjacent carbons, to make a pi bond (π bond). These combine to give a π-bond H H C H C H C H H H H H H C H C C H Alkynes Same idea as above, but only 2 substituents for the σ orbitals. Only 2 atomic orbitals needed (1 x 2s, 1 x 2px) → 2 molecular orbitals 2 x sp2). H C C H 180o e.g., ethyne Note, there is still one electron in each of the py and pz orbitals (→ total of 2 σ bonds and 2 π bonds on each carbon) These combine to give a π-bond py H C C H + H C C H pz H C C H Other atoms participate in the same type of hybridization. See the examples below. sp 2 sp 3 O N H sp 3 sp 2 H O H N sp More Alkane Nomenclature Rules for naming compound 1) take longest linear chain (this gives “root”) 2) number the molecule from one end to get lowest substitution numbers 3) name and number substituents 4) if more than one substitutent, use di, tri, tetra 5) arrange substituents in alphabetical order (excluding prefixes such as di, tri, i.e., triethyl precedes methyl) Nomenclature other functional groups The groups that are arranged in alphabetical order are: halogens (Cl chloro, Br bromo, I iodo), NH2 amino, CN cyano, NO2 nitro, and alkyl groups: methyl CH3 (Me), ethyl CH3CH2 (Et), propyl (Pr) C CH3 H CH3 CH3CH2CH2, isopropyl (iPr) Rotational isomers Bond rotation along σ-bonds takes place readily at room temperature. However, not all “twisted” structures are of equal energy. Generally, the most stable structures have big groups as far away from each other as possible. The repulsion of groups are called van der Waals interactions. Let’s look first at a simple structure – ethane. 60 o H H H H H H Staggered 0o H H HH H H Eclipsed Your eye H H H H H H H H H H H H When there are more groups, the situation is a little more complex 0o CH3 H3 CCH 3 HH H 60o H Eclipsed H CH3 H HH H CH3 H CH3 120 o H H H H CH3 180 o H CH3 H Staggered GAUCHE Eclipsed CH 3 CH3 Your eye H H H H Staggered ANTI CH3 H H H CH3 H CH 3 H H H CH3 CH 3 H H H H H CH 3 Other functional groups take a different priority (highest priority at top, lowest at bottom and then the “alphabetical groups” after that. 1 O Carboxylic Acid Always C1 1 C H 3C 2 OH O Ketone 2-propan “one” (acetone) C H 3C 3 CH3 O Aldehyde C H 3C ethan “oic acid” (acetic acid) propan “al” (1propanal) 1 H 4 Alkene H3CHC=CH2 5 Alkyne H3CC≡CH 1-prop “ene” (double bond starts at carbon 1) 1-propyne (triple bond starts at carbon 1) 1) lowest # most important - find longest chain, arrange substituents in alphabetical order and lowest substituent # e.g. CH3 C H CH3 H 3C H C Cl longest chain = 4 = butane 2) numbers, if same numbers, irrespective of which end you begin with, choose alphabetical and lower i.e., 2-chloro-3-methylbutane not hendecagon 3-chloro-2-methylbutane Br Cl C C C H 3C H C H H2 CH3 2 CH3 ∴ 4-bromo-2-chloro-4-methylhexane is correct 3-bromo-5-chloro-3-methylhexane is not: the lowest number is higher in this name Order of preference for naming Highest R RCOOH carboxylic acid > RCHO > R > R3C-OH > RNH2 > R2C=CR'2 > RC≡ ≡ CR' O aldehyde ketone alcohol amine then come other substituents: halo, methyl, nitro, etc. * note that this order is opposite in some books Other Functional Groups ETHER CH3CH2OCH3 ethyl methyl ether AMINE CH3NHCH2CH3 (CH3)3N ethylmethylamine trimethylamine NITRO NO2 CH3CH2NO2 nitroethane (1-assumed) NITRILE C≡N CH3CN ethanitrile alkene alkyne* O ESTER H 3C R O CH3CO2CH3 methyl ethanoate Cyclic Alkanes (CnH2n) Just add cyclo to name. cyclopropane cyclobutane cyclopentane cyclohexane Substitutents on cyclic systems – geometric isomers. H 2C HC H2 C H2 CH3 C H 2C CH2 HC C H CH 3 CH2 CH CH3 CH3 CH 3 CH3 CH3 CH3 cis-1,2-dimethylcyclopentane trans-1,2-dimethylcyclopentane SAME SIDE of plane defined by ring OPPOSITE SIDES Alkenes ANE → ENE Naming ends is ene H2 C C H2 H C C H H2 C ethane H3C-CH3 → ethene H2C=CH2 CH3 3 - heptene longest chain with double bond in it - gives double bond lowest possible number i.e., 3-pentene not 4-pentene NB bond strength of double bond (~267 kcal/mol) less than bond strength of single bond (350 kcal/mol) Therefore C=C is more reactive than C-C. However, for a given structural isomer, there may be two geometric isomers that are not interconvertible @ RT (you would have to break the bond to interconvert them). GEOMETRIC ISOMERS If the groups with highest atomic number are on the same side - Z-isomer (zusammen), if on opposite sides – E-isomer (entgegen) H C H C H 3C CH3 H C CH3 C H H 3C Z-isomer E-isomer If 2 identical atoms go to next atom in chain; next structure is Z-1-bromo-2-pentene (put starting carbon of alkene at lowest number of chain) Br H2 C H2 C CH3 C H C H Cyclic Alkenes cyclobutene 4-methylcyclopentane Triple bond – Alkyne ANE → YNE Cl H 3C C C Cl CH CH 3 4-chloro-2-pentyne Alcohol ANE → ANOL yne ending Br Br OH C H 3C H C H2 OH CH 3 4-bromo-2-pentanol Ketone ANE → ONE (“OWN”) Cl O ONE has precedence over other groups listed above 3-chloro-2-butanone Aldehyde ANE → ANAL 4-methylpentanal H O Carboxylic Acids → ANE ANOIC ACID ETHANOIC ACID (acetic acid) 3-BROMO-PROPANOIC ACID O O OH Br Summary of Formulas & Isomers OH 1. Molecular Formula C4H8 C3H8 These are clearly different 2. Structural isomers: Same molecular formula – different arrangement of groups, Stereoisomers have different properties i.e., boiling point, melting point, etc. e.g., C4H9Cl 2-chlorobutane 1-chlorobutane Cl Cl Cl Cl 2-chloro-2-methylpropane 1-chloro-2-methylpropane 3. Stereoisomers – geometric isomers Cyclic alkanes Alkenes Cl Br Cl Br cis-1-bromo-2-chlorocyclopropane E-2-pentene trans-1-bromo-2-chlorocyclopropane 4. Rotational isomers 0o 3 H 60o CH3 H3 CCH HH E-2-pentene H Eclipsed H CH3 H HH H CH3 H CH3 120 o H H H H CH3 180 o H CH3 H Staggered GAUCHE Eclipsed CH 3 CH3 Your eye H H H H Staggered ANTI CH3 H H H CH3 H CH 3 H H H CH3 CH 3 H H H H H CH 3 Alkanes: Properties and Reactions CnH2n+2 b.p. m.p. Increasing London Forces (going down table) CH4 -164 -182.5 C2H6 -88 -183 C3H8 -42 -190 C5H12 36 -130 C10H22 174 -30 Polyethylene burns 140 (C 100H202) Tg~20 the chemistry of parent defines chemistry for the series Homologous series each “homologue” 1 CH2 more Natural gas = methane and some ethane, a little propane Petroleum (black gold, texas tea) Distillation gives Natural gas pet ether Ligroin light naptha Gasoline Kerosene Loading oil oil general paraffin nor. Asphalt C4 C5 - C6 C7 C5-C9 C6 - C12 C12-C15 C15-C18 C16 - C20 < 20o 30-60 60-90 85-200 200-300 300-400 >400 Alkanes – other natural sources - “fart” produced in anaerobic bacterial decomposition (e.g., cow stomach (blue angels)) also found in salt mines, coal mines Reactions of Alkanes As we already saw, not very reactive 1) 2) strong bonds C-C, C-H not polar C - C no polarization C - H small polarization hard to start reactions. Reactions generally happen at “functional group (C=O, C-N, C≡C, etc.) Alkanes - cyclo alkanes “paraffin” means unreactive H2S O4 No reaction KMnO4 No reaction Na No reaction To decompose Na use alcohol in parafin – only the alcohol reacts 2Na + 2 CH3CH2OH 1 → 2 CH3CH2ONa+ + H2 Halogenation with Cl2, or Br2 H H H Br 2 Br + HBr This is a radical chain reaction – doesn’t work in the dark 3 parts Initiation, Propagation, Termination hν Br 2 2 Br Initiation endothermic H Br H H + HBr Br + STEP 1 Propagation endothermic H H + Br 2 Br STEP 2 net reaction exothermic Termination 2 Br H Br 2 + H H H + Br Br Let’s look at a simpler system CH4 + Cl2 → CH3Cl + HCl ∆Hrxn = -104 kJ mol-1 1) Cl2 → 2 Cl• ∆H1 = +243 kJ mol-1 2) Cl• + CH4 → CH3• + HCl ∆H2 = +4 kJ mol-1 3) CH3• + Cl2 → CH3Cl + Cl• ∆H3 = -108 kJ mol-1 But for bromination CH4 + Br2 → CH3Br + HBr ∆Hrxn = -34 kJ mol-1 1) Br2 → 2 Br• ∆H1 = +192 kJ mol-1 2) Br• + CH4 → CH3• + HBr ∆H2 = +66 kJ mol-1 3) CH3• + Br2 → CH3Br + Br• ∆H3 = -100 kJ mol-1 ex othermic Note that chlorination is mildly endothermic in the first step of propagation (step 2) whereas bromination is quite endothermic. In the second step of the propagation, both are exothermic. The overall reaction rate is dependent upon the activation energy in the slowest step (step 2). The Ea for chlorination is much lower that for bromination, which one might predict from the overall enthalpies of the steps. The overall reaction with Cl2 faster than Br2 If excess halogen, e.g., Cl2, more chlorination ie. CH4 → → CH3Cl CH2Cl2 But for I2 step 1 very endothermic + 200 kJ/mol ∴ reaction very slow so not useful For F2, steps 1 and 2 are very exothermic step a ~ -144 kJ/mol → explosive reaction ∴ use other reagents to make F- alkanes (CoF3, SF4) If one uses unsymmetical alkanes, there are different types of CH bonds. Depending on which bond reacts, different products are formed. The preference depends on the strength of the C-H bonds and on the number of hydrogens of a given type. In general, effects from both factors are observed. The bond strengths of CH bonds depend on the number of carbons connected to the central carbon. Reactant Products H3CH H3CCH2H (Bold carbon is primary) (H3C)2CHH (Bold carbon is secondary) (H3C)3CH (Bold carbon is tertiary) H3C• H3CC•H2 a primary radical (H3C)2C•H a secondary radical (H3C)3C• a tertiary radical The ease of forming a carbon radical (and the order of highest stability) is 3° (tertiary) > 2° (secondary) > 1° (primary) > methyl Recall for chlorination (and bromination) RH + Cl• → R• + HCl endo (slow) R + Cl2 → RCl + Cl• exo What happens in a molecule with both types of hydrogens – both happen Bond Dissociation kJ mol-1 426 405 397 376 H2 C H 3C H2 C k1 CH3 CH2 H3 C Cl Cl2 H3C H2 C C H2 Cl a PRIMARY alkyl radical k2 Cl H 3C Cl2 CH CH3 H 3C H2 C CH2 Cl a SECONDA RY alkyl radical The rates are proportional not only to the bond strength of the CH bond being broken, but also on the statistical number of hydrogens. The bond strength is the most important factor. (i.e., generally k2 > k1) Rate of reaction via 1° CH ∝ k1 [CH3CH2CH2CH3][Cl•] x fn 6 H’s Rate of reaction via 2° CH ∝ k2 [CH3CH2CH2CH3][Cl•] x fn 2 H’s Where fn is some fractional effect of arising from the statistics. The product ratio between these two products will be rather similar to k1 / k2 The actual ratio of products is 1° alkyl halide 45%, 2° alkyl halide 55% 2 Alkanes - Combustion This is the fundamental reaction of the 20th century CnH2n+2 → (3n +1)/2 O2 H4C + 2 O2 → CO2 → + 2 H2O + ∆ e.g., cigarette lighter C4H10 + 6.5 O2 Alkane CH4 H3CCH3 C4H10 → 4CO2 + 5H2O ∆H combustion (kJ/mol) 213 373 687 656 NOTE Ring strain (C 4H8) C5H12 nCO2 + (n+1) H2O + heat 845 793 (C 5H10) 4.1 A Alkyl Halides ? Haloalkanes Chemistry controlled by bond polarity δδδ+ C-Hδδ+ C-XδX = F high dipole moment → X = I lowest polarity ∴ alkyl halides have higher m.p. & b.p.’s than related alkanes that only have London forces CH4 b.p. -164 °C H2CCl2 b.p. 35 °C H3 C H2 C Lewis acids attack here: X C H2 H +, Ag+, BF3 Lewis bases (nucleophiles) attack here: -OH, :NH 3, -SH, -CN Preparation of Alkyl Halides 1 Haloalkane Preparations; From alkanes (Review) Seen above H Cl Cl2 hν or ∆ 3 From alcohols, OH 2-propanol HBr Br a substitution reaction 4 Addition to alkene Br Br 2 very fast Br Reactions of Alkyl Halides 5 Nucleophilic Substitution NC- + NC I + I Many examples of substitution reactions. (For OH- need dilute solutions, see below) 6 Elimination of HX I + hot + conc. HO- I + H2 O base H 7 Reactions with Group 1 or 2 metals Li / Mg δ+ Mg δBr δ- δ+ MgBr O ether = Organometallic SN2 Reactions (5) NaI + Br δδ+ 2-propanone I + NaBr (ppt) Kinetics process of process - d[EtBr]/dt = k[EtBr]1[I-]1 Second order – bimolecular; these kinetics are observed for MeX, 1° RX (RCH2X) and 2° RX (RR'CHX) BUT NOT FOR 3° RX (RR'R"CX) Called SN2 “substitution nucleophilic bimolecular” Go from 1 isomer to a different isomer; i.e., Inverted stereochemistry at the reaction centre Most nucleophilic substitutions take place this way. NCBr NC O trans cis Mechanism of the SN2 Reaction _ HO- H H HO C H H Br C H H Br H HO C H + Br H The SN1 Reaction – another substitution reaction As we saw above, 1° and 2° alkyl halides mostly undergo SN2 reactions. For 3° alkyl halides, however, need very polar solvents and non-basic nucleophiles to observe nucleophilic substitution. However, the kinetics are different. (CH3)3CBr + - N3 Azide faster in → polar solvent Rate Law (Experimental !) Rate = k[RBr]1 (CH3)3C-N3 a first order reaction; implies the overall reaction is not an elementary step (that the rate detemining step doesn’t need a nucleophile). slow Br 1 2 ionization + C + - N3 C + + Br N3 Why doesn’t the SN2 happen with 3° alkyl halides? Two reasons – i) Steric reasons (the space the nucleophile needs, to attack the carbon, is occupied by other groups). The alkyl groups block backside attack. ii) the 3° cation is sufficiently stable that another reaction pathway exists. Order of stability of carbocations (just as we found for radicals): 3° ((CH3)3C+) > 2° ((CH3)2C+H) > 1° (H3CC+H2) > H3C+ Why? The alkyl groups help stabilize the cation by donating electronic charge. Smearing out charge is energetically favourable. One can accelerate the rate of these reactions using Lewis acids. Silver removes the chloride in tertiary chlorides to help form the cation. This is a classic Lewis acid: Lewis base reaction, Cl + Ag + + C Ag Cl Ag Cl ppt Nu Nu This is a useful chemical test for 1° 2° and 3° alkyl halides Some nucleophilic substitutions H2O OH Cl + HCl water is nucleophile and solv ent alcohol EtOH O Cl + HCl alcohol is nucleophile and solv ent ether HOOH Cl Cl Cl alcohol NC- CN RNH2 N R + R nitrile KOH H N R amine R Cl ammonium salt Summary of Nucleophilic Substitution SN2 relative rate Methyl CH3X 30 1° C-CH2X 1 2° C2CHX 0.03 3° C3CX 1 x 10-6 SN1 H3C+ never see C-CH2+ almost never see 1-10% of SN2 rate 100% Elimination (6) - We saw above that OH + an alkyl halide gives an alcohol (an SN2 reaction). This is true only if COLD DILUTE OH (KOH, NaOH) is used. If HOT CONCENTRATED (e.g., > 3M) is used, a second order ELIMINATION occurs to give an alkene. OHH HOT H2O Br + CONC KOH In this reaction, the HO- is acting as base, not a nucleophile. The attack of at the carbon is slower than the attack of the base at the H - Rate Law; Rate = k[RX][OH ]; called E2 (elimination bimolecular) Bond Making Br H H H R R H H H H R H H HObond breaking Generally the more carbon groups on a double bond, the more stable it is: Saytzeff’s rule Cl + NaOH heat + major Bromo-3-methylbutane minor 3 methyl butane Organometallics (7) We can completely alter the electronic distribution in a molecule by converting an alkyl halide into an organometallic compound. H3Cδ -Iδ + Mg → H3Cδ--Mgδ+-I methylmagnesium iodide + - Br Li ether + + Li LiB r These are carbanions (very strong bases and nucleophiles). This is one of the few reactions we learn in year 1 from which C-C bonds can be formed. THIS IS AN IMPORTANT REACTION! 8 Reaction of Organometallics with water (the carbanion is a strong base) δ- δ+ MgBr + H OH ether H + HOMgB r pKa ca. 50 pKa 15.5 9 Reaction of Organometallics with ketones or aldehydes (IMPORTANT C-C bond formation #1) δ- δ+ MgBr +R δ+ δO + H3O ether R R R OMgBr R R ketone OH alcohol 10 Reaction with CO2 (IMPORTANT C-C bond formation #2) + δ- δ+ MgBr +O δ+ δO H3O ether O OMgBr pH ca. 2 O OH c arboxylic acid Alkenes Preparation of Alkenes From Haloalkanes (6) CH3 OK Br HEAT H 1-bromo-3-methylbutane 3-methyl-1-butene This is the E2 mechanism we described above 11 From alcohols an alcohol + an dehydrating acid (H2SO4, H3PO4) OH + ∆ distill H3PO4 catalyst cyclohexanol b.p. 156 °C + catalysts H2O cyclohexane b.p. 82 °C We shall discuss this below. Alkene Reactivity Dominated by π bonds π bond ∴ energy (~ 267 kJ/mol strength of the double bond) more reactive than a σ -bond (310 kJ/mol); It is a Lewis base - attack by E+ on π-electrons, i.e. In plane of screen above or below π-bond H H R H H H3O + H H C R R + H H R H H H X X H R H H H NOT H R H + C H H X H H H R H H X These are extremely reactive almost as reactive as the metal CHECK HYPERCHEM Fats: in the body: triglycerides O O O O O O beef fat lard human leving corn olive C12 0 0 1 0 0 0 C14 0 1 3 5 1 0.1 C16 27 24 27 14 10 7 C18 14 9 8 3 3 2 1 Oleic C18 49 47 48 0 50 84 2 linoleic C18 2 10 10 0 34 5 3 limolenic C18 0 30 0 0 Other important alkenes: squalene → cholesterol; β-carotene → retinal (vision) β-selinene, celery oil myrcene, bay leaf oil α-pinene, cedar leaf oil Alkenes may exist in two different geometric isomers (see above) (Z- and E-) Z-2-butene E-2-butene This is the source of black/white vision Opsin H H O N BODY hν H Vitamin A BODY OH N H β-carotene Alkene Reactions 12 Electrophilic additions Remember that the order of cation stability is: 3° ((CH3)3C+) > 2° ((CH3)2C+H) > 1° (H3CC+H2) > H3C+ additions to alkenes usually proceed via the most stable cation. H H HX C X + X + X = OH, need H as catalyst X = B r, Cl, I get both cis and trans addition Bromination This is a special case of electrophilic addition. H Br R H H R Br2 Br R Br + Br H R Br - Can also use ICl (I+ Cl ) Only get trans addition OH I Cl + ICl + H H2O + C Cl I H H Br2 Cl Cl Br H Br + Br Br + Br Br Why do Br2 and ICl add trans but HCl adds randomly: The answer: Br and I are Lewis bases 13 Reduction In organic chemistry, addition of H’s or removal of oxygen is “reduction”. The removal of hydrogens or addition of oxygen is “oxidation”. Conversion of C=C to HC-CH is, therefore, a reduction D H2 D D Pt catalyst H H D cis-addition of H2, the reaction happens at the solid surface of Pt Process used commercially to “hydrogenate” fats. Oleic acid O H m.p. 4 oC O Pt H2 O H m.p. 70 oC O stearic acid Stearic acid is a saturated fat; Not so good for you. Better for your health are polyunsaturated fats; they are more easily processed. Alcohols & Ethers Unlike the functional groups we have seen so far, alcohols and ethers (to a lesser extent) are polar molecules. In the case of alcohols, there is strong H bonding and reasonably large dipole moments. CH 3OH O H O H H O H O pKa ca. 16 methanol consumption can lead to blindness ethanol: in beer, wine, liquor 2-propanol (isopropanol) rubbing alcohol Alcohols Preparation SN2 of alkyl halides with hydroxide (5) I + HO dilute - OH + cold I Hydration of alkenes (12) OH H2SO4 + H 2O Dilut e H 2O Addition of organometallics to aldehyde or ketone (8) H δ+ δ- δ+ MgBr Mg δBr O H OMgB r O ether + H3 O H OH electrophilic C CH3I + 2 Li Nucleophilic C → CH3Li + LiI From electronegativity, the polarization (and reactivity) of a ZM bond, M = Na, Li, Z = first row elements follows the reactivity CH3 > -NH2 > -OH > -Cl - - (NOTE: This is consistent with our discussion of acidity. It’s harder to make the H3C than Cl ) Any reaction that converts one of the compounds on the left to one on the right will be thermodynamically favoured. e.g., CH3Li + H-OH → CH3-H + LiOH Less stable More stable CH3MgBr H-OH → + CH3-H + BrMgOH Other C-C Bond Forming Reactions δ- δ+ MgBr +R R δ+ δO H3O ether R R + OMgBr R R ketone OH alcohol Note: CAN’T DO THE FOLLOWING REACTION. δ- δ+ MgBr + R R Br ether R R H alkyl halide Ethers are less polar than the alcohols (No OH’s for H-bonding) They are made in a similar fashion to alcohols Preparation of ethers (5 SN2) + Br + CH 3O- Na O + NaB r CH 3 14 Making Alkoxides (Reducing Metals) 2 OH + 2Na 2 ONa + H2 Alkoxides are strong bases (stronger than hydroxide) and also good Lewis bases or nucleophiles Recall: 2HO-H + 2K → 2HO-K + H2 ↑ Reactions of Alcohols Preparation of alkyl halides (3) Alcohols & halogen acids OH + e.g. HCl, HBr, HI Br HBr + strong acid H2 O weak acid Note that the reaction doesn’t work under basic conditions OH Br + Br weak base + HO- strong base + Na The mechanism H OH + + HBr O Step 1 + H H + O Step 2 H + SN2 Br Br Alcohol & Dehydrating Acid (11) e.g. 80% H2SO4, or H3PO4 is required H O H H 2SO4 - H 2SO4 H + O + C H H HSO4 H + H 2O + H2 O Br cyclopentanol Elimination 15 Alcohols + Oxidizing Agents One can use inorganic salts to oxidize (remove H’s or introduce oxygen) onto organic molecules. eg. CrO3 or Chromium IV or K2Cr2O7/H2SO4 Na3Cr2O7/H2SO4 ≡ H2CrO 4 Chromic acid or KMnO 4 (MnVII) O Cl + N Or “Organic Chromium Salts, like pyridine chlorochromate (PCC) 1o Alcohol (a) with PCC OH C - CH2OH PCC CH 2Cl 2 H removes this H (b) with O Ethanol (acetaldehyde) K2Cr2O7/H+ or KMnO 4 H OH H + KMnO 4 O H 2o Alcohol + Any oxidant OH H 2-propanol O H H + + Na2 CrO7 H2SO 4 O 2-propanone Cr O OH KMnO4 O 3o Alcohol no reaction at normal temps, No H-COH to oxidize! Aldehydes and Ketones Preparation Oxidation of 1o Alcohol (15) OH H + PCC CH2 Cl2 O H 2-methylpropanol 2-methylpropanal Oxidation of 2o Alcohol (15) H + KMnO4 O OH 3-methyl-2-butanol 3-methyl-2-butanone Reactions of Aldehydes and Ketones Nucleophilic Addition The carbon in C=O is electron-poor, an electrophile. δ+ δO O Y + ionic X δ- δ+ X Y OR O X covalent Y Examples Organometallic + Ketone (8) aldehyde → secondary alcohol ketone → alcohol Ph δ+ δBr Mg δ- δ+ MgBr O Ph OMgB r O ether + H3 O Ph OH 16 Reduction Ph with NaBH4 NaBH 4 O Ph O CH3 OH H 17 Reduction with H2 and a Pt catalyst NOTE: This is exactly the same as addition of H2 to an alkene Ph Ph H2 O 18 Pt-catalyst EtOH solvent O H Reaction with N-Compounds H N e.g., hydrazine H2NNH2; 2,4-dinitrophenylhydrazine O 2N NH2 NO2 ; hydroxylamine H2NOH O + H 2NNH2 + H 2NNH2 N in Et OH O O O + N H2 H + N + N H2 + NH 2 H2 O NH2 OH NH2 NH NH2 N NH2 NH2 + H2 O 19 Oxidation - Aldehydes Occurs very easily, while can use strong oxidants such as K2Cr2O7/H+ or KMnO 4 Can also use weak oxidizing agents eg. Ag O O H OH KMnO4 + + MnO2 Another more relevant oxidant – how to make a mirror OH O HO H Ag HO HO HO HO HO HO NOTE: Ketones + oxidants OH O HO HO + HO OH O + HO HO Ag mirror HO No reaction at normal temps Carboxylic Acids Structures: Pure glacial acetic acid Methanoic acid (formic acid) O O OH H OH Ethanoic acid (acetic acid - 5% soln in H2O is vinegar) The Structure Type O R Y Is very common R = any alkyl or aryl (aromatic) e.g. R O O O R NH2 Amides O R OMe Esters Cl O Acid chlorides acetic anhydride Preparation of Carboxylic Acids Organometallic + CO2 (10) δ- + Li δ+ O δ- + Li O ether C δ+ O O + H 3O pH 2 OH O (2-Methylpropyl)lithium Aldehyde + most oxidants (19) H O 2-phenylethanal H3O + O + OH Na2 Cr 2O7 O 2-phenylethanoic acid (Phenyl acetic acid) 1o Alcohol + Any oxidant H H OH + OH KMnO4 O ethanol + MnO2 ethanoic acid (acetic acid) 20 Reaction of -COOH with an Alcohol → Esters Need a strong acid catalyst OH + O H H3O O + + H2 O O OH ethanoic acid ethyl ethanoate (ethyl acetate) (this is related in mechanism to reaction 18) Reaction is a Nuc. Add followed by an elimination (2) Acidity pKa≈ 3- 5 O OH + + H2 O + H3 O O O ethanoic acid (acetic acid) -two O atoms and therefore the O-H bond is weakened moreover, there is resonance through which the charge is dispersed. O O O O (Boy have we already covered this) Compare with Alcohols O OH + H2 O Only one O to share electrons pKa ≈ 16-18 + + H3 O NOTE: an alkane C-H is an unbelievably weak acid, nothing to stabilize the charge CH4 → ie. pKa of x 50 CH3- + H3O+