Nuggets of Knowledge for Chapter 4 Alkanes Chem 2310

Transcription

1 I. Intrductin t Alkanes Nuggets f Knwledge fr Chapter 4 Alkanes Chem 2310 Alkanes are hydrcarbns cntaining nly single bnds. We will study them first because they frm the framewrk n which all ther types f rganic cmpunds are made. Straight-chain alkanes: The simplest alkanes are the straight-chain alkanes. Their names and cndensed frmulas are given belw. Yu will need t learn these names. The mlecular frmula fr these alkanes is C n H 2n+2. number f C's cndensed frmula name 1 CH 4 methane 2 CH 3 CH 3 ethane 3 CH 3 CH 2 CH 3 prpane 4 CH 3 CH 2 CH 2 CH 3 butane 5 CH 3 (CH 2 ) 3 CH 3 pentane 6 CH 3 (CH 2 ) 3 CH 3 hexane 7 CH 3 (CH 2 ) 3 CH 3 heptane 8 CH 3 (CH 2 ) 3 CH 3 ctane 9 CH 3 (CH 2 ) 3 CH 3 nnane 10 CH 3 (CH 2 ) 3 CH 3 decane Branched alkanes: The carbns in a straight-chain alkane can be rearranged t frm branched cmpunds with the same number f carbns and hydrgens. Cmpunds which have the same mlecular frmula but a different bnding sequence are called cnstitutinal ismers. Cyclic alkanes: Carbn atms can als be jined t frm a ring, but in this case tw less hydrgens are needed, making the mlecular frmula C n H 2n. The names f these cmpunds are frmed by adding cycl t the beginning f the name f the straight-chain alkane. Cyclic alkanes can als have cnstitutinal ismers, but they must have a ring f sme size. Bicyclic alkanes: Alkanes can als cntain mre than ne ring. If the rings share ne r mre carbns, the cmpund is called a bicyclic alkane. The carbns that are shared by all f the rings are called bridgehead carbns.

2 If the tw rings share nly ne atm, it is called a spir cmpund (these are nt very cmmn). If the tw rings share tw carbns (an edge), it is called a fused cmpund (these are quite cmmn). If the tw rings share three carbns (tw edges), it is called a bridged cmpund (these are als fairly cmmn). Fr every ring that is frmed, the mlecular frmula lses tw hydrgens. II. Physical Prperties, Surces, and Uses f Alkanes Physical Prperties f Alkanes Alkane mlecules are attracted t each ther nly by weak van der Waals frces. They have very lw surface tensin, and are nt sluble in water. They are less dense than water. Alkanes with higher masses have higher melting pints and biling pints. They increase abut 30 C fr each CH 2 grup added. Alkanes with mre branches have slightly lwer melting pints and biling pints than straightchain alkanes because they have less surface fr the van der Waals frces t act n. This change is smaller than the change that ccurs with mass. Alkanes which are rigid and symmetrical ften have unusually high melting pints (but the biling pints are unaffected). This is because they will pack mre efficiently in a crystal lattice, and it takes extra energy t break them apart. In general, alkanes with 1-4 carbns are gases, alkanes with 5-17 carbns are liquids, and alkanes with 18 r mre carbns are slids. Surces f Alkanes Mst alkanes cme frm petrleum. After being pumped ut f the grund, petrleum is distilled t give the fllwing mixtures: temperature number f carbns prduct less than 30 C 2-4 petrleum gas C 4-9 gasline C 8-16 kersene C diesel C heavy il (heating il, jet fuel) ver 300 C with vacuum petrleum jelly, paraffin wax residue ver 35 asphalt

3 Anther surce f hydrcarbns is natural gas. It cntains abut 70% methane, 10% ethane, and 15% prpane. The ethane and prpane are remved and sld separately; what is sld as natural gas is nearly pure methane, which burns very cleanly. Uses f Alkanes Mst f the alkane mixtures btained frm petrleum and natural gas are burned t release energy. Gasline and diesel pwer cars and trucks, natural gas heats hmes, and s n. Unfrtunately, this may be cntributing t glbal warming, and alternatives are being researched. Alkanes are als used as raw materials in the manufacture f ther rganic chemicals such as plastics, pharmaceuticals, and fertilizers. If all f the petrleum reserves are used fr fuel, we will have difficulty making these imprtant prducts. IR Spectrscpy f Alkanes Alkanes have nly tw typical sets f IR bands. Since nearly all cmpunds have these bands, they are imprtant t knw. Yu can tell if it is an alkane by making sure nne f the ther bands are present. C-H stretching: There are several types f C-H stretching with very similar frequencies. These give rise t the C-H bands between 3000 and 2850 cm -1. CH 2 bending: There is a CH2 scissr vibratin arund 1460 cm -1, and CH 2 and CH 3 rcking vibratins arund 1380 cm -1. III. Reactins f Alkanes Alkanes are quite stable, and nly underg a few reactins. This is why lcatins n rganic mlecules where nitrgen r xygen are bnded are called functinal grups this is where reactins ccur, rather than n the alkane-like part f the mlecule. There are three cmmn reactins f alkanes. All f them usually give mixtures, and tw take place under cnditins f high temperature and pressure. They are nt cmmnly used in the labratry fr synthesis. Cmbustin: All alkanes are flammable, and will cmbine with xygen t frm carbn dixide and water when ignited with a spark. If insufficient heat r xygen are available, carbn mnxide, elemental carbn (st), r hydrcarbn fragments can be frmed. This reactin prvides pwer fr much f ur sciety. It can be used t generate electricity, and als makes cars, trucks, planes, trains, and bats g. This reactin is als a majr hazard. When wrking with alkanes, it is imprtant

4 t remember hw easily they can be ignited, and hw easily it can get ut f cntrl. Hydrcracking: Large alkanes can be cleaved at high temperatures in the presence f special catalysts t frm smaller alkanes that are mre useful. Halgenatin: In the presence f light r heat, hydrcarbns react with chlrine gas r liquid brmine t prduce alkyl halides. Multiple prducts are usually bserved; these can be separated by fractinal distillatin, which is nly practical n an industrial scale. Flurine is nt used because it is s energetic the reactin becmes explsive, while idine is nt used because the reactin is t slw. Because f the multiple prducts created, this reactin is nt usually useful in the labratry. Hwever, we will see in future chapters hw certain starting materials give limited prducts, therefre making them mre useful. IV. Nmenclature f Alkanes The rules fr naming rganic cmpunds have evlved ver time. The current rules are set frth by IUPAC the Internatinal Unin fr Pure and Applied Chemistry. The purpse f these rules is t make it s that n matter hw a cmpund is drawn, it will have the same name, and that if yu are given a name, yu can determine the structure. The three main steps in naming a cmpund are: Step 1: Identify the principle chain (r ring) this gives the rt name f the cmpund. Step 2: Number the principle chain (r ring) this gives us a label fr each carbn s the lcatin f substituents can be given. Step 3: Name and rder the substituents this tells us what is attached t the main chain. Step 1: In alkanes, the principle chain (r ring) is the lngest cntinuus chain f carbns, r the biggest ring. Rings and chains are cnsidered separate carbns that are part f a ring cannt be part f a chain. If there are tw lngest chains f equal length, r if there is a chain and ring f equal length, chse the ne with the mst substituents. If a chain and a ring cntain the same number f carbns, and there are n substituents, chse the ring.

5 Step 2: Alkanes are numbered by fllwing the rules belw: Rings with nly ne substituent dn t need t be numbered. Branched alkanes must be numbered frm ne end f the principle chain r the ther, nt frm the middle; cyclic alkanes can be numbered starting anywhere in the ring. Always give the lwest number t the first substituent. If the first number is the same in tw different pssible ways t number, give the lwest number t the secnd substituent (and s n). If there are tw substituents n the same carbn, they cunt separately when determining the lwest number. If tw different ways f numbering give identical numbers, give the lwest number t the substituent that cmes first in the alphabet. Step 3: Each grup cnnected t the principle chain is cnsidered a substituent. Each substituent is given a name and a number, and listed by alphabetical rder befre the rt name f the cmpund. If there is mre than ne f the same substituent, list them tgether, using the prefixes di (fr tw), tri (fr three), tetra (fr fur), etc. The prefixes are nt used t alphabetize. Separate tw numbers with a cmma, and numbers frm letters with hyphens. Dn t leave any spaces. Alkyl substituents are named by replacing the ane at the end with yl. There are a few that have cmmn names that must be memrized. Straight-chain alkyl substituents are methyl, ethyl, prpyl, butyl, etc. Cyclic substituents are cyclprpyl, cyclbutyl, cyclpentyl, etc. Sme substituents have cmmn names, but these names are becming less cmmnly used. The cmmn names are isprpyl, isbutyl, sec-butyl, and tert-butyl. In these last tw, the b is used t alphabetize, nt the s r t. Branched substituents are named as a substituent n a substituent n a principle chain. The numbering f the substituent chain always begins at the pint f attachment. Parenthesis ( ) are used arund the name f the branched substituent if a number is needed t specify where the whle substituent is n the main chain.

6 IV. Cnfrmatins f Straight and Branched Alkanes Mlecules underg many types f mtins: the mve thrugh space, they tumble as they mve, their bnds vibrate (as we saw in IR), and they als rtate (r twist) arund their single bnds. In this sectin, we will discuss this rtating mtin. As a mlecule rtates arund a single bnd, the varius psitins that it can have are called cnfrmatins. These cnfrmatins can have different energies. T simplify matters, we will lk at ne bnd at a time. We will lk dwn this bnd t see what relatinship the atms cnnected t each f the nes in the bnd have. T represent this, we will draw Newmann prjectins: a circle represents the tw atms in the bnd, atms attached t the frnt atm in the bnd are drawn with lines cming ut f the center, and atms attached t the back atm in the bnd are drawn with lines cming ut frm the circle. If the atms are n tp f each ther, ffset them a little when yu draw them. The angle between tw atms n different ends f the bnd is called the dihedral angle. As the mlecule rtates, yu can lk at hw the dihedral angle changes, and hw this changes the energy f the mlecule. When all the atms in a bnd have 0 dihedral angles, they are said t be eclipsed. When all the atms in a bnd have 60 dihedral angles, they are said t be staggered. When a mlecule is in a staggered cnfrmatin, it has lwer energy than when it is in an eclipsed cnfrmatin. This is because the electrn cluds f the atms repel each ther (called steric hindrance). When tw larger grups have a 60 dihedral angle, this is said t be a gauche interactin. It makes the mlecule have higher energy. When tw larger grups have a 180 dihedral angle, they are said t be anti t each ther. This desn t add energy t the mlecule. V. Stability f Cyclalkanes The amunt f energy in different mlecules can be cmpared by lking at the heat f frmatin f each mlecule (the energy needed t g frm the elements t the cmpund). Straight-chain and branched alkanes all have abut the same amunt f energy per carbn, but sme f the cyclic alkanes have higher energy than thers. Cyclprpane has the highest energy, fllwed by cyclbutane. Cyclpentane, cyclheptane, and cyclctane energies are slightly higher than straightchain r branched alkanes.

7 Cyclhexane is has the lwest energy, and is the same as a straight-chain r branched alkane. There are tw reasns that a ring may have higher energy. Angle strain ccurs when putting the atms int a ring frces the angle f a tetrahedral atm t be less than 109. This is mst cmmn in small rings. Trsinal strain ccurs when putting the atms in a ring frces the atms attached t them t be eclipsed, rather than staggered. The mlecule will try t twist t relieve this pressure. This can be an issue in small and large rings. VI. Cnfrmatins f Cyclhexane Cyclhexane rings are particularly cmmn in nature because f their stability. Fr this reasn we will study their cnfrmatins in mre detail. A cyclhexane ring is usually represented by a hexagn. This is what the mlecule lks like frm the tp. Each carbn has ne hydrgen atm ging up, and anther hydrgen ging dwn. These are represented by using wedges fr the bnds cming tward yu, and dashes fr the bnds ging away frm yu. T accurately represent the cnfrmatin f cyclhexane, it must be drawn in frm the side. This is called the chair cnfrmatin, and it shws the angles f the carbn and hydrgen atms mre realistically. When this infrmatin is imprtant, we will use the chair cnfrmatin instead f the hexagn. A chair cnfrmatin has ne hydrgen n each carbn ging up, and ne ging dwn, just like in the view frm the tp but the angles are nt all the same. The nes which are ging straight up r straight dwn are called axial, while thse that are slanted up r dwn at an angle are called equatrial. Each carbn must have a hydrgen ging up and a hydrgen ging dwn. Likewise, each carbn must have an equatrial hydrgen and an axial hydrgen. Ging arund a ring, ne carbn will have axial-up and equatrial-dwn, and the next ne will have axial-dwn and equatrial-up, and s n. There are actually tw chair cnfrmatins fr each cmpund. A cmpund will flip back and frth between these tw cnfrmatins. When it des s, all f the axial hydrgens will becme equatrial, and all f the equatrial hydrgens will becme axial. Hwever, they will maintain their rientatin in the up r dwn directin. Cyclhexane rings with substituents ther than hydrgen can als be written in either type f representatin. T cnvert the hexagn representatin t a chair cnfrmatin, yu must first determine whether the substituent is ging up r dwn based n the wedged r dashed bnd. Then chse a carbn n the chair cnfrmatin, and determine whether the up r dwn

8 directin is equatrial r axial n that carbn. If there is mre than ne substituent, cunt carbns until yu reach the next psitin and repeat. The cnfrmatin in which a substituent is in the equatrial psitin has lwer energy than the cnfrmatin in which it is in the axial psitin. The axial psitin has higher energy because the substituent bumps int the ther axial substituents, and because it has a gauche interactin with the ring. When there are multiple substituents, the mst stable cnfrmatin will be the ne with the mst substituents in the equatrial psitin. When there are the same number f axial and equatrial substituents, the cmpund with the larger substituent in the equatrial psitin will have lwer energy. The tert-butyl substituent is s bulky and sterically hindered that it cannt ccupy an axial psitin. The mlecule will try t twist that way, encunter the steric hindrance, and g back. Since these rings nly have ne cnfrmatin, smetimes they are said t be lcked in that psitin.