It is makrooh to pray facing a person’s face
Question

                Can I pray if there is someone sitting in a chair in front of me and I can only see his back?.
Praise be to Allah.There is nothing wrong 
      with praying behind someone who is sitting if he gives you his back; 
      rather it is makrooh to pray facing his face, if that will distract the 
      one who is praying. 
Ibn Qudaamah (may Allah 
    have mercy on him) said in al-Mughni (2/40):
It is makrooh to pray 
    facing a person’s face, because ‘Umar disciplined people for doing so. 
    According to the hadeeth of ‘Aa’ishah, the Prophet (blessings and peace of 
    Allah be upon him) used to pray level with the middle of the bed whilst I 
    was lying down between him and the qiblah. When I needed something, I would 
    not want to get up facing him, so I would slip away. Agreed upon. End 
    quote. 
Al-Nawawi (may Allah have 
    mercy on him) said in al-Majmoo‘ (3/230): It is makrooh to pray with 
    a man or woman in front of you, facing him where you can see him. This was 
    regarded as makrooh by ‘Umar ibn al-Khattaab and ‘Uthmaan ibn ‘Affaan (may 
    Allah be pleased with them). And because it usually distracts one. It is 
    also makrooh to look at what will distract one, such as a garment with 
    decorations on it, or looking up towards the sky, and other things that are 
    mentioned in the saheeh hadeeths. Al-Bukhaari said in his Saheeh: 
    ‘Uthmaan (may Allah be pleased with him) regarded it as nakrooh to face a 
    man when praying. Al-Bukhaari said: Rather that applies if one will be 
    distracted by that, but if he will not be distracted, Zayd ibn Thaabit said: 
    I do not mind; a man does not interrupt the prayer of another man.
End quote. 
With regard to one who is 
    sitting with his back towards the one who is praying, if he does not move 
    and distract the one who is praying, there is nothing wrong with praying 
    behind him, just as there is nothing wrong with praying facing one who is 
    sleeping, because of the report narrated by al-Bukhaari (382) and Muslim 
    (512) from ‘Aa’ishah, the wife of the Prophet (blessings and peace of Allah 
    be upon him), who said: I used to sleep in front of the Messenger of Allah 
    (blessings and peace of Allah be upon him), with my legs in the direction of 
    his qiblah. When he prostrated he would poke me and I would draw my legs up, 
    and when he stood up I would stretch them out. She said: There were no lamps 
    in the houses at that time. 
And Allah knows best.