CHEM Week 2 notes

Transcription

1 EM Week 2 notes Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, hemistry, 2007 (John Wiley) ISBN: You should now be able to Understand the basis of drawing organic structures onvert between a condensed molecular formula and a skeletal or line structure Determine the formula of a molecule from its skeletal representation Know and recognise the functional groups Be able to name an alkane 1

2 FIRST YEAR EMISTRY EM1002 Tutorials Start in week 1 omplete the assignment sheet after working through the critical thinking problems in the tutorial Laboratory Work Starts in week 2 check your timetable for your session Assessment 15% laboratory assessment (see first lab session for details) 15% tutorial quizzes (3 per semester: weeks 5, 9 and 12) 10% spectroscopy assignment (begins week 4 with a deadline of week 7) 60% 3 hour exam at the end of semester Naming Alkanes: Stems and Substituents Alkanes have the ending -ane The stem is given by the number of carbons Stemane methane ethane propane butane pentane hexane heptane octane nonane decane To name substituents just replace the ending -ane with -yl Alkane Alkyl substituent propane propyl 3 2 pentane pentyl

3 Locates the substituent(s) in the molecule Nomenclature ow do you name an organic compound? Need a unique name for every unique compound The name has several parts: The number of carbons in the longest chain containing the functional group Number(s) Substituent(s) Stem Ending The Rules The parts of the molecule that are not the stem The functional group present 1) Find and name the longest carbon chain containing the fuctional group - this is the stem - and add the ending 2) Identify substituent(s) 3) Number the longest chain to give the lowest possible numbering for substituent(s) 4) Allocate a number to every substituent 5) List substituents in alphabetical order 6) Identical substituents are indicated by prefixes: di (2), tri (3), tetra (4), then penta (5), hexa (6)...then write it all out as one word anching ut 2-methylpropane 2-methylbutane 2,2-dimethylbutane 3-methyl-pent-2-ene 2,4-dimethylhexane Find the longest chain to work out your prefix If multiple bonds are present, your longest chain must include them Use the methyl, ethyl, propyl etc to indicate branches + 2- etc to show where Useful older names 5-methyl-hex-2-yne Isopropyl- tert-butyl

4 Functional Groups Functional groups are the interesting bits of a molecule s and s form the skeleton multiple bonds and heteroatoms are the action centres 2-butyne benzene cyclohexene cholesterol 3 Aspirin

5 alcohol alkene carboxylic acid arene ester ether alcohol amine amide amine

6 Functional Groups Alcohol -ol Propanol Ketone -one Butanone Aldehyde -al Ethanal arboxylic Acid -oic acid exanoic acid Ester -oate Methyl butanoate N 2 Amide -amide Butanamide Functional Groups Ether -ether Diethyl ether N 2 Amine Amino- or amine Ethyl amine l Alkyl chloride * hloro- or chloride 2-hlorobutane l Acid chloride * -anoyl chloride Propanoyl chloride * Similarly for F, & I: alkyl / acid fluorides, bromides & iodides

7 What have we done to date? You should now be able to: 1. Understand the basis of drawing organic structures 2. onvert between a condensed molecular formula and a skeletal or line structure 3. Determine the formula of a molecule from its skeletal representation 4. Know and recognise the functional groups 5. Be able to name an alkane Next: Isomerism Isomerism 1. What are isomers? 2. onstitutional Isomers 3. onformational Isomers 4. Double bond Isomerism

8 Isomerism ow many different compounds are there with the formula 2 2 l? * ow do we tell them apart? ow do we name them? ow are they different? Do they have different properties? * (Excluding those with l or bonds) uch! There are 16 different compounds 2 2 l l 2 l l l 2 l l l l l l l l l l l l

9 Isomerism Same molecular formula but different structures Isomers same molecular formula onstitutional Isomers Different nature/sequence of bonds Stereoisomers Different arrangement of groups in space onformational Isomers Differ by rotation about a single bond onfigurational Isomers Interconversion requires breaking bonds Enantiomers Non-superposable mirror images Diastereoisomers Not mirror images Isomerism Same molecular formula but different structures Isomers same molecular formula onstitutional Isomers Different nature/sequence of bonds Stereoisomers Different arrangement of groups in space onformational Isomers Differ by rotation about a single bond onfigurational Isomers Interconversion requires breaking bonds Enantiomers Non-superposable mirror images Diastereoisomers Not mirror images

10 onstitutional Isomers Isomers which differ in nature or sequence of bonding! Within a homologous sequence of alkanes, the number of constitutional increases rapidly n : n 2n : No. of onstitutional iosmers : onstitutional formula AND Question Draw and name the constitutional isomers of methylpentane 3-methylpentane hexane 2,3-dimethylbutane 2,2-dimethylbutane

11 Isomers have Different Properties The physical and chemical properties of constitutional isomers may be very different, particularly when different functional groups are present For example the molecule with formula 4 8 may be a ketone, aldehyde, alkene/ether or alkene/alcohol Question: which is which? ketone alkene/ether aldehyde alkene/alcohol Isomerism Same molecular formula but different structures Isomers same molecular formula onstitutional Isomers Different nature/sequence of bonds Stereoisomers Different arrangement of groups in space onformational Isomers Differ by rotation about a single bond onfigurational Isomers Interconversion requires breaking bonds Enantiomers Non-superposable mirror images Diastereoisomers Not mirror images

12 Stereoisomers Isomers which differ in arrangement of groups in space (same nature and sequence of bonding) Two groups: onformational isomers (conformers) differ by rotation about a single (-) bond not normally separable at room temperature onfigurational isomers Interconverted only by breaking and remaking bonds. This process normally requires considerable energy and does not happen at room temperature Back to the Isomer Tree Isomers same molecular formula onstitutional Isomers Different nature/sequence of bonds Stereoisomers Different arrangement of groups in space onformational Isomers Differ by rotation about a single bond onfigurational Isomers Interconversion requires breaking bonds Enantiomers Non-superposable mirror images Diastereoisomers Not mirror images

13 onformational Isomers Use ethane as an example ( 3 3 ) Sawhorse representation Newman projection eclipsed staggered rotate back carbon 60! In straight chain alkanes, rotation about - rapid at R.T.! Differences in energy arise from steric interaction Energy Dihedral angle 12 kj/mol Barriers to Rotation

14 Barriers to Rotation Example: butane onformational Isomers Energy kj/mol +16 kj/mol +4 kj/mol 0 kj/mol Dihedral angle Back to the Isomer Tree Isomers same molecular formula onstitutional Isomers Different nature/sequence of bonds Stereoisomers Different arrangement of groups in space onformational Isomers Differ by rotation about a single bond onfigurational Isomers Interconversion requires breaking bonds Enantiomers Non-superposable mirror images Diastereoisomers Not mirror images

15 onformational Isomers Straight chain alkanes! Rotation around each - bond readily occurs! onformational isomers result yclic alkanes! Rotation is restricted within a ring! Since rotation would require atoms to pass through the ring very high energy barrier ycloalkanes Various isomerism possible! onstitutional: l l l l l l! onfigurational: l l l l Don t need to have double bonds to have configurational isomerism!

16 ycloalkanes cis and trans! These structures are diastereoisomers or diastereomers! They have different physical and chemical properties! alled cis and trans to distinguish them l l l l cis-1,2-dichlorocyclopentane l l l l trans-1,2-dichlorocyclopentane ne More Look Isomers same molecular formula onstitutional Isomers Different nature/sequence of bonds Stereoisomers Different arrangement of groups in space onformational Isomers Differ by rotation about a single bond onfigurational Isomers Interconversion requires breaking bonds Enantiomers Non-superposable mirror images Diastereoisomers Not mirror images

17 Question Identify the isomerism: constitutional conformational diastereomeric Will continue with diastereomers and enantiomers in a later lecture Isomers Resulting from Structural Rigidity yclic alkanes! Rotation around the - bond within a cycloalkane is restricted compared to that of a hydrocarbon chain onsequently disubstitute cycloalkanes occur as: onstitutional isomers: Diastereoisomers: l l l l l l Different sequence of bonding l l l l Differ only in stereochemistry

18 Alkene diastereoisomers A "-electron overlap requires a fixed geometry around the bonded carbon atoms A double bond is contains both a!- and a "-bond Pi-bonds result from p-orbital overlap Electron density concentrated above and below plane Rotation around the - axis requires breaking the "-bond (~128 kj mol -1 ) does not occur at room temperature No rotation about the two ends! - bond " - bond onsequence of restricted rotation about the double bond 3 3 melting point = -139 º boiling point = 4 º (Z)- 2-butene Different compounds! 3 3 melting point = -106 º boiling point = 1 º (E)- 2-butene

19 Alkene diastereoisomers! If one end of the = bond has same two groups not a diastereoisomer l l l l All examples of the same molecule, flipping whole molecule gives superimposable structures Double Bond Isomers! If both ends of the = bond bear two different groups A! B A B X Y X! Y! Then TW stereoisomers are possible called diastereomers and labeled Z or E (Although A or B can be the same as X or Y) Different compounds!

20 Nomenclature Z and E The rules:! Z/E determined by assigning a priority to each of the pairs of groups on each carbon of the double bond! The higher the atomic number of the atom attached, the higher the priority! If identical atoms attached, work along the chain until the first point of difference, then go by atomic number! If high priority groups on same side of = plane # Z double bond (German: zusammen, together)! If high priority groups on opposite sides of = plane # E double bond (German: entgegen, opposite) (Z) Different compounds! (E) Double Bond Isomers igher priority groups on the same side of double bond alkene is denoted (Z) igher priority groups on opposite sides of double bond alkene is denoted (E) l (Z)- 2-butene higher priority groups on same side (E)- 2-butene higher priority groups on opposite sides (E)-1-bromo-2-chloropropene higher priority groups on opposite sides

21 Nomenclature of Alkenes Find longest chain containing double bond - this gives the stem For alkenes the ending is -ene (instead of -ane for alkanes) Give the double bond the lowest number possible Name the substituents as usual Determine if the double bond is E or Z Name the following alkenes: Note: Alkynes have no diastereomers Geometry of both carbon atoms is linear There is only one way to attach two substituents in a straight line! Pop Quiz Name these molecules fully: A... B D... What is the isomeric relationship between: A and B.. and D..

22 Pop Quiz Name these molecules: A...1-hexene... B...(E)-3-hexene......(E)-1,3-hexadiene... D...(Z)-1,3-hexadiene... What is the isomeric relationship between: A and B onstitutional isomers and D Diastereoisomers... Summary You should now be able to 1. Describe alkane conformational isomers 2. Understand the difference between constitutional isomers and stereoisomers 3. Recognise constitutional, conformational and diastereomeric (cis/trans) isomers 4. Name isomeric structures correctly Next! rganic Reactions!

23 rganic Reactions Four general types of organic reaction Acid base reactions (See later: arboxylic acids) - loss and gain of protons ( + ) Substitution reactions - one group replaces another Addition elimination reactions - group(s) added or taken away xidation reduction reactions - loss and gain of electrons Two types of reagent Two key types of reactant: Electrophiles: electron loving, chase negative things ( or $ ) # positive (+ or $ + ) themselves eg. +, l Nucleophiles: nucleus loving, chase positive things (+ or $ + ) # negative ( or $ ) themselves eg. -, -, - N, N 3..

24 Pop Quiz lassify these reagents as nucleophile or electrophile N N + + nucleophile + N( 3 ) N( 3 ) 2 nucleophile + l electrophile l urly Arrows urly arrow indicates electron pair movement A curly arrow starts at the electron pair that moves and ends at the atom to which the electron pair has moved Arrow from a bond # bond breaks Arrow between two species # new bond formed between them R R +

25 quiz Add curly arrows to complete this mechanism N N + + Remember: urly arrow starts at the electron pair that moves urly arrow ends at atom to which the electron pair has moved Arrow from a bond # bond breaks Arrow between two atoms # new bond formed between them quiz N + + N Remember: urly arrow starts at the electron pair that moves urly arrow ends at atom to which the electron pair has moved Arrow from a bond # bond breaks Arrow between two atoms # new bond formed between them

26 Reminder: Alkene Structure A double bond has two potions: ne! bond (sp 2 + sp 2 overlap and one " bond (p + p overlap) The electron density of the " bond lies above and below the plane of the double bond! - bond " - bond all six atoms lie in one plane Addition to an Alkene General reaction: Reactions of alkenes occur at the weak and accessible "-bond Addition occurs with replacement of a " bond and a A-B! bond with two! bonds: energetically favourable + A B A B A-B could be 2, l 2, 2, l,, I, 2 (-)

27 Alkene Reactions 1. ydrogenation (addition of hydrogen) 3 3 Pd/ catalyst alogenation (addition of a halogen) Alkene Reactions 3. ydrohalogenation (addition of a hydrogen halide) ydration (addition of water) S 4 catalyst 3 3 +

28 .. Two steps ydrohalogenation! Step 1: + adds An unstable intermediate carbocation is formed 3 3!! + step a carbocation +! Step 2: - adds The carbocation is intercepted by bromide anion 3 3 a carbocation + step alogenation As the halogen approaches, it is polarised by the alkene! Step 1: l + adds ne end of the polarised halogen reacts with alkene l l 3 3 +l!! l 3 3 l + l! Step 2: l - adds The carbocation is intercepted by chloride anion 3 3 l + l l 3 3 l

29 ydration Addition of water: + catalyst required (eg., dilute 2 S 4 ) Again two main steps but also a third mini-step Step 1: + adds to give a carbocation step carbocation Step 2: 2 intercepts (using spare electrons on ) 3 3 carbocation + water molecule step S 4 - step Step 3: + is removed (by conjugate base of 2 S 4 ) + 2 S 4 Question Draw the products of these electrophilic addition reactions I + I dil 2 S 4

30 Product Isomers? When X adds to a symmetrical alkene: only one product + l l l (Z)-2-butene 2-chlorobutane But X addition to unsymmetrical alkene two possibilities l l + l + 2-methyl-2-butene 3-chloro-2-methylbutane 2-chloro-2-methylbutane Markovnikov s Rule ne product dominates l l + l + Minor product Major product Markovnikov s Rule: The hydrogen of an unsymmetrical reagent adds to the end of the double bond that has the greater number of hydrogen atoms already directly attached No 's attached 3 3 Two 's attached but why?

31 arbocations The intermediate in the reaction is a carbocation! arbocations have 6 electrons and a positive charge! All carbocations are unstable! But some are more unstable than others arbocation stability a tertiary alkyl carbocation a secondary alkyl carbocation a primary alkyl carbocation a methyl carbocation more stable least stable Markovnikov s Rule Major product results from more stable carbocation intermediate 3 l 3 3 step l minor product 3 l less stable carbocation more stable carbocation l 3 3 l major product 3

32 Pop Quiz Predict the major products of these reactions: / (dil 2 S 4 ) I + I ydrogenation atalyst required (eg Pd on charcoal) to break strong - bond + 2 Pd/ Pd/ 3 3 Both s added to same face of = Mechanism more complex ( 2 adsorbed onto Pd surface)

33 Quiz 2 / Pd catalyst 2 dil 2 S 4 Answers 2 / Pd catalyst dil 2 S 4 2

34 Benzene Benzene Benzene does not behave like 1,3,5-cyclohexatriene It is about 140 kj mol -1 more stable than predicted It is less reactive than expected 2 2 without catalyst no reaction Further evidence for the unusual structure of benzene comes from the bond lengths: carbon-carbon bond - (alkane) = (alkene) benzene Bond length 154 pm 133 pm 139 pm Bond strength 356 kj/mol 636 kj/mol 518 kj/mol Structure of Benzene There are 2 bonding models used to describe the structure of benzene 1) Valence bond model Each of the atoms in benzene is sp 2 hybridised, forming! bonds to two neighbouring atoms and a! bond to one atom. Each carbon also has a p orbital which can form a " bond. This gives alternating double and single bonds There are 2 possible representations with the " bonds in different positions These structures are related by movement of electron pairs and are resonance forms.

35 Structure of Benzene 1) Valence bond model continued The bonding in benzene is best described as an average or 'resonance hybrid' of the two bonding arrangements The resonance hybrid will have : alf a!-bond between each adjacent pair of carbons A bond strength and bond length between that of a - single bond (no!-bond) and a = double bond (one!-bond) Resonance - recap from semester 1 Resonance structures differ only in the position of their!-electrons Atoms do not move in resonance structures Resonance forms must be valid Lewis structures (i.e. only 8 electrons for, N, ; only 2 electrons for ) The more resonance structures possible for a given molecule, the more stable the molecule. Structure of Benzene 2) Molecular rbital Model The 6 p-orbitals (one from each carbon atom) form part of a delocalised!-bond system characteristic of aromatic compounds. The electron delocalisation results in a stabilisation of 150 kj/mol compared to the hypothetical cyclohexatriene structure: Blackman Figure This results in clouds of electron density above and below the plane of the benzene ring. Localised electrons = constrained to one atom or shared between two atoms (e.g. in a "-bond) Delocalised electrons = shared between three or more atoms Delocalisation of electrons leads to greater stability

36 Benzene Benzene might look like 3 x = s but! The reactions of benzene are not like an alkene at all 2 Fe 3 (catalyst) + Fe or Al 3 can also be used as the catalyst! Substitution rather than addition is observed! And a catalyst is required! Benzene has special properties: it is aromatic! 6 x " electrons (i.e., 3 x =) in a ring # compound is aromatic Requirements for Aromaticity yclic system; ring of atoms (nearly) planar onjugated (sideways overlap of p z orbitals 4n+2 "-electrons in cyclic, conjugated system n=0 or integer 1, 2, 3 etc. ence 6, 10,14 "-electrons

37 Recap You should now be able to: " Distinguish between conformational and configurational isomers of alkanes " Identify alkene diastereoisomers as E or Z " Identify electrophiles and nucleophiles " Give the major products obtained from the reaction of alkenes with electrophiles (remember Markovnikov s rule!) " Understand why benzene does not react like an alkene